Them being the oldest at the age given is also highly unlikely. Despite what any of the books might say about the average age and lifespan of a Magus, there have been multiple people who have written and run simulators since AM5 first came out. You can find old threads talking about them, along with a link to the files if you want to run it yourself. What those simulations show is that the actual "average" life expectancy of a Magus is around 150 years post gauntlet, with the longest lived 10% around 190 post gauntlet. The top 1% are into the 200's post gauntlet.
The numbers from the simulation might actually be low for some saga since the average cause of death for older Magi is aging roles, with Final Twilight being minimal. While 1 in 10 aging rolls explode and 1 in 100 double explode, in a simulation running thousands of Magi (the total that have ever been in the Order, not just the number around in 1220) your outliers will not get any explodes (or very low one) and actually bump up against the age limit of their longevity ritual. The simulator tends to top out at around a LR lab total of 100 (200 years), with additional decades coming from lifestyle and familiar cords.
The written blocks of AM5 not lining up with the actual numbers is not something new or limited just to the life expectancy of a Magus. In the case of the age of a Magus, I know Ben McFarland (EDIT: I mistakenly said Timothy Ferguson) popped up in a thread talking about aging and was surprised when he ran the numbers when adding in lifestyle, lab health, and the familiar bronze cord. From his reaction it would seem that those were not normally taken into account when writing the age of a Magus. On average those modifiers should add half a century, with the potential of a century plus.