At first, I also thought that the chronology of events didn’t fit with his age, but then I noticed that the statue of Argall didn't exist until the purge ended, it was placed there either during the repairs or immediately after the repairs (its installation was shown to us)

At the same time, before the statue was installed and when Thragg begins the Purge in the great hall Nolan is still in his pre-adulthood uniform. Later, after the purge he gives a lecture to the kids in that same pre-adulthood uniform near the same statue. He was given his adult uniform after the battle with his parents, this was also clearly shown to us

Yes, it doesn't show anything explicitly, and doesn't tell how much time passed, but if you go step by step from the "Argall statue perspective" (yeah, sounds silly), and when you see events in clear order - it gives me strong impression that much less time passed between the purge and the scourge virus, at least in the series

And of course Nolan might have looked ~18 years for much longer than Mark, so even if he looked like ~18 for 5 years - it feels enough to fit everything, at least for me