Well really my concern isn't so much how fast the character ages as how slow it matures and how to handle this with the character creation rules.
The issue is the how character creation assumes your 'early childhood' is from 0 to 5, and then gives you xp for that and for every year afterwards. Combine this with the Nephilim writeup saying they reach adulthood at roughly the age of 80, and without making any aging rolls the 'starting' version of a Nephilim has a huge ammount of xp to spread around... but technically is only just an adult.
Also the player desires this Nephilim to be a Redcap, of Mythic Companion level, to occupy the 'slot' of his Mage character. I don't object, I actually find that an interesting concept and fitting into the mold of the 'Heroic' style of mythic companion redcaps... it is just how to resolve it with character creation etc. My desire, since I am the only experienced Ars player in the group, was to shoot for a group of newbie just gauntleted magi. So, these guys are roughly 20 years old with no post-gauntlet time.
Admittedly the Nephilim, being made at roughly 100 years now and thus with a Divine Might of 5, is an easy kill for a Perdo Vim spell. Right now, as it is a mythic companion, I'm kind of ignoring the huge disparity in exp with his character as I think comparatively that is his only area to really shine. ( Such as him electing to lay enough exp into 'Bows' to have an 8. )
But how do you keep them somehow in line with the group over time? The Mages are going to outpace him rather quickly in the powers and penetration department, and his might advances 'very' slowly. So while I feel like he is getting a ridiculous ammount of xp to begin with, his lack of powers and/or might is going to hamper him in the long run.
Also problematic is figuring out when is appropriatte to say the character may have had his redcap apprenticeship, and then started accrueing his additional levels of magical items. The standard mage apprenticeship age of five, with him being 100 years old, gives him a stupendous ammount of item levels, even more when allowing for his selection of the magical item virtue from true lineages. Perhaps a reasonable age is 80ish, when he may have become active/adventurous and encountered his eventual Mercere recruiter/teacher/master.
All in all so far the Nephilim feel like a cool idea with some problematic execution ( or at least unaddressed areas ) in using it with other PCs.