I have a character in a RP that is a Ex-Psyjic. He is a Bosmer who is 400 years old, but looks like he's in his 40s. As the topic title asks, what school of magic would have the most impact of delaying age and halting cell aging?

Restoration I'd bet.

Restoration and Necromancy.

Necromancy and chronomancy.

I'd agree on Resto.. although it'd probably end in disaster anyways. Look at the Augur of Dunlain (a Resto specialist himself).