NASA says Olympus Mons is 100 times larger by volume than Earth’s largest volcano, Mauna Loa, and identifies it as a shield volcano. The article does not give a specific 3.5-billion-year formation date for Olympus Mons; instead, it discusses ancient volcanic activity on Mars more generally, including eruptions over a 500-million-year period in Arabia Terra.

NASA’s Mars resource page identifies Olympus Mons as the largest volcano in the solar system. The page emphasizes its enormous size and geologic significance, but the visible description does not provide a precise statement that it formed about 3.5 billion years ago.

The report discusses the age of the Olympus Mons basal scarp and states that "the age of formation of the basal scarp of Olympus Mons is somewhat uncertain" but suggests that this feature may have formed "3.5 to 4.0 b.y. ago." It further notes that during the next billion years, volcanic activity spread and built much of the edifice, implying an origin in the Late Noachian to Early Hesperian billions of years ago.

Crater count dating of volcanic constructs in Tharsis, such as Olympus Mons, indicates that their main edifices are Hesperian in age, having formed roughly 3.5–2.5 Gyr ago. However, young lava flows with Amazonian ages, in some cases less than 100 Myr, are superposed on these edifices. This combination of ancient construction ages and much younger surface flows implies that the large volcanoes such as Olympus Mons began forming more than 3 billion years ago and remained intermittently active for most of Martian history.

NASA’s descriptive resource on Olympus Mons notes that the giant shield volcano formed by repeated lava flows over a long period of Mars’ history. It emphasizes that crater counting shows the volcano has been active for much of Mars’ past and that the youngest lava flows are tens of millions of years old, but it does not attribute a specific 3.5‑billion‑year age for its initial formation.

The generated model age of ~3.67−0.10/+0.05 Ga places a constraint on the initiation of significant volcanism and flexural loading at Olympus Mons. On the basis of the paleo-flexure considerations, we conclude that the bulk of Olympus Mons formed after this time. ... The oldest crater retention age was found for Sulci Gordii, of ~3.53−0.28/+0.09 Ga, in agreement with previous estimates. Using the assumption that the aureole deposits represent flank failure from stresses within the edifice, this age represents the time at which Olympus Mons had reached a significant fraction of its present-day size. From the crater retention ages derived from the topographically discordant lava flow and the aureole deposits, we conclude that the primary volcanic construction of Olympus Mons occurred between 3.67–3.53 Ga.

Given the current lack of samples acquired from known locations on Mars, planetary scientists have to estimate the age of the surface by counting the number of visible craters: a higher number and density of craters indicates older terrain. Hesperian (3.7–2.9 billion years ago)… Amazonian (2.9 billion years ago to present). Most estimates place [the Amazonian] start date at about 2.9 billion years ago.

Olympus Mons, largest known volcano in the Solar System, is about 26 km high, more than 600 km across at base, and surrounded by a well-developed scarp. Crater counts on the volcano and its associated deposits indicate that construction began in the Late Noachian to Early Hesperian and continued, probably episodically, into the Amazonian. Thus, the edifice as a whole has a formation history spanning more than 3 billion years, with its earliest construction dating back to roughly 3.5–3.7 Ga.

Olympus Mons is a giant shield volcano that rises about 22 kilometers above the surrounding plains. Crater counting of the oldest units associated with the volcano shows that its initial construction dates back more than 3 billion years, to the transition between the Noachian and Hesperian periods of Martian history. Younger lava flows on its flanks, however, are much younger, some only a few million years old, indicating very long-lived volcanic activity.

Crater-count ages of units around Olympus Mons suggest that major construction of the volcano occurred in the Late Noachian–Early Hesperian, with basal units and aureole deposits yielding ages around 3.5–3.6 Ga. Subsequent volcanic activity built up the shield through the Hesperian and Amazonian, with some flank flows dated to less than 100 Ma. Thus, while the onset of Olympus Mons formation is ancient (on the order of 3.5 Ga), volcanism continued much more recently.

Space.com reports that Olympus Mons has taken billions of years to form and notes that some regions may be only a few million years old. The article supports a long formation history, but it does not pin the volcano’s origin to exactly 3.5 billion years ago.

The Tharsis Montes volcanoes and Olympus Mons are giant shield volcanoes. Although estimates of their average surface age have been made using crater counts, the length of time required to build the shields has not been considered. Crater counts for the volcanoes indicate the constructs are young; average ages are Amazonian to Hesperian. In relative terms; Arsia Mons is the oldest, Pavonis Mons intermediate, and Ascreaus Mons the youngest of the Tharsis Montes shield; Olympus Mons is the youngest of the group. Depending upon the calibration, absolute ages range from 730 Ma to 3100 Ma for Arsia Mons and 25 Ma to 100 Ma for Olympus Mons. Previous crater counts for Olympus Mons calderas and lower flank flows reveal volcanic activity clustered around 100–200 Myr ago and as young as 2.5 Myr ago. On Olympus Mons, averages over large areas give characteristic ages of a few hundred Myr, but youngest flows have ages in range 5–50 Myr.

Top: Olympus Mons (18.5°N, 133.2°W) is mapped with six calderas numbered according to elevation, lowest elevation is 1, next higher is 2, etc. The work confirms previous findings by others that volcanism was continuous throughout Martian geologic history until about one to two hundred million years ago… and the latest large-scale caldera activity ended about 150 million years ago in the Tharsis province. Robbins and coauthors suggest a transition from explosive to effusive activity occurred at different times for the Martian volcanoes, but say that generally the transition was made around the Hesperian–Amazonian boundary.

Tying Mars' crater-count ages to specific ages in years requires you to make informed assumptions about how often impacts occurred at different times in the past. The modern chronostratigraphic time scale for Mars is pegged to the number of craters of different sizes that are present on the areas of different deposits… Uncertainty about when impacts of different size were happening at different rates produces pretty huge ranges in estimates for the ages of some of Mars' historical events. The worst is the beginning of the Amazonian period, which might have happened anywhere from 3.5 to 2 billion years ago, depending on whom you talk to.

An abstract on the evolution of Olympus Mons notes that the volcano shows a complex history with units mapped from Hesperian to Amazonian ages based on crater counts. It indicates that the construction of Olympus Mons began early in Mars' history and continued into relatively recent times, implying a total lifespan of billions of years for the volcanic system but not fixing a unique age such as 3.5 Ga for its origin.

EBSCO’s overview says Olympus Mons formed over the course of billions of years and that its last eruption is thought to have been about 25 million years ago. The source supports a prolonged construction history, but it does not specify a 3.5-billion-year start date.

Rice University summarizes Olympus Mons as a massive shield volcano built by many lava flows over billions of years. This supports the idea of an ancient, long-lived construction process, but it does not provide a precise age of 3.5 billion years.

According to current estimates, this extinct shield volcano formed during Mars' Hesperian Period (ca. 3.7 to 3 billion years ago), which was characterized by widespread volcanic activity and catastrophic flooding. Based on their findings, Hildebrand and his team theorize that the upper rim of the 6 km (3.7 mi)-high main escarpment surrounding Olympus Mons was likely formed during the late Noachian to early Hesperian (ca. 3.8 to 3.5 billion years ago) by lava flowing into liquid water.

Crater counts from HRSC images of the northwestern flank of Olympus Mons indicate that several lava flows are very young in geological terms. Model ages for these flows range from about 115 Ma down to as young as 2 Ma. These very young ages for surface units on the flanks of Olympus Mons imply that the volcano remained active long after its initial construction phase in the Late Noachian/Early Hesperian, which is dated at around 3.5–3.7 Ga for the aureole deposits and basal units.

In planetary geology, Olympus Mons is generally understood to have been built over a very long span of Martian history, with many surface units much younger than the volcano’s earliest construction phases. A claim that it "formed about 3.5 billion years ago" is often a simplification of early volcanic activity on Mars rather than a precise single formation date for the entire edifice.