iunno man they've never been very clear about how he works. by the end of the show he's 17 and that's dang old for a dog but he seems as spry as ever. And yeah he's magic, but Jermaine isn't, and he's 17 too. He has access to a lot of arcane crap I suppose but he doesn't seem the type to artificially extend his life on the other hand these are commissions so it's best not to take any of it too seriously, he's mixing canons all over the place.
I'm not quite a fan of where some of what the finale or the writers went with "canon" after ward left if you ask me. Comics and early season have a much brighter vision of the future and most of this is directly inspired by comic future (which itself has several different iterations).
Thats the beauty of AT that I think comic writers caught well. The show doesnt have to be the only story. The fun should never end there.
I think some of the iterations has Jake live at least to the time Finn is a young adult. I wanted to have an older Jake here as the comics had it with yellower skin and longer jowls but that ended up being harder to draw so we just tried to make him fat.
Only so much you can add on an already great picture! And I disagree the best part of AUs is trying to think about things seriously. Its what makes AT a great show by tickling the imagination!