the only way a magus MiA makes sense to me is if you add 10 years to the minimum age for an MiA and figure they did the education after apprenticeship (or a combination of after and before).
Sure, unless the parens in question values education of his apprentice more than having a lab- or scribal slave. Some magi want apprentices for the end result more than for the free labour,
Or if the parens is Magister in Arbitus himself?
But yes, as SIlveroak wrote, it would be easier to assume the two training periods weren't at the same time. This requires the magus to be older, but also messes less with his status and reputation.
I would also consider that your maga has to spend 2 seasons per year teaching because of a mundane academic title. Which means she won't be teaching for her whole life, as a magus lives waaaay longer than a mundane, and having the same professor at a university for 150 years would be extremely suspicious, to say the least. So if you want to leverage the field xp-wise, you can rule that after 30 years of teaching she would have to feign her retirement for the mundane scholarly world, to avoid rising suspicions regarding "that scholar that never ages". 30 years sounds like a reasonable span mechanically, considering the xp numbers, and also in-world as it would be hard to hide her non-aging for longer than that.
I'm playing an Ex Misc Magus with Magister In Arbitus at the moment and that big lump of XP at the start is very welcome. Although in practice the kinds of things you'll spend it on (languages, academic abilities) are much more flavourful than powerful. Still, having artes liberales and philosophiae at 5 plus a couple of languages doesn't hurt at all.
The major downside is not the teaching since almost certainly your magus isn't going to continue to teach during the saga, its the age. My magus started at age 38, 3 aging rolls in before the game even started. And all of them without a longevity potion.
I think its a great virtue though. My Ex Misc comes from a tradition much like the Learned Magicians, very academic, starts with Hermetic Numerology. I'm enjoying playing him as a fussy, somewhat naive professor type.
I found the Fountain of Youth through my research. I suggest you go through your Herodotus again...