I want to roleplay an old mage, but the protagonist is only 20. How can I make this work?

I want to play an old mage who is incredibly intelligent, incredibly wise, but nuttier than squirrel droppings.

I got a portrait ready which goes really well with either an invoker or diviner:



I got my sound file ready:



I came up with a name that sounds good, and is harmonious seeing as how he's a disciple of Gorion:

Gideon.

So I create the character, roll a 94 (hard to do even for specialist mages), and set the stats:

10
18
16
18
18
14

I go into the biography to create Gideon's backstory:

Aw crap, that's right.

20 years.

And adding a "1" before the 20 makes no difference because the opening narration states the protagonist's age as being 20 years old.

Is there some kind of lore-friendly, D&D-friendly justification why a human wizard would be 20 but look 80? Now of course I know I could make him a wild mage, seeing as how in PnP 2nd Edition wild surges can change one's age. But this is NOT true of an invoker. Yes, haste can artificially age a character, but I see no reason why he would've used haste, considering it doesn't effect spell casting and he would have expeditious retreat for increased movement speed.

So what do I do with this character concept? Scrap it?

I really wish the Baldur's Gate team would've allowed the player to set their character's age. I see no reason why it would necessarily have to be set at 20 in all circumstances. For instance, sometimes a wizard apprentice will remain in their master's tower for decades upon decades before striking out on their own.

What a bummer.