The way I see it, the reason so many characters are seemingly 'defaulted' into being a teen, or young adult, is because most roleplayers feel that there's more to be done with the life of a younger character;
They can find a mate (which is a big one), they can create their own family, they're old enough to fight, and still have the youthfulness to be resilient to most injuries and still have some energy left over. They see it as the prime of their character's life, and therefore it gives them the upper paw in whatever situation they may run into.
Which, if you've just made a character, but don't know much about them (except for maybe a quickly-drawn up past bio about their family and leaving the pack/pride), this would seem to be the way to go. I see this happening a lot in some less experienced roleplayers, but it's certainly not a nub-RPer thing to do, to create a 'young buck' sort of character.
Therefore, when someone runs into a character that's not around that age (2-3 years old), it seems almost appalling that someone would make their character older and [gasp] 'weaker', in their eyes. Four, five years old is hardly anything for real-life people and animals, but in a roleplay situation, where most characters don't last too long (due to storylines or the RPer growing tired of the same character), it seems unusual, and it may just seem that the character is close to dying, when they really aren't.
So, I think it's just a misconstrued idea of what age is (thus, the title of this thread, which I really like, Lady). xD
I do enjoy an older character, though.
They're unique, and usually have a blossoming past to them. In some cases, I've found that mid-RP, the character will seem to 'act on their own', and somehow reveal to me something special about themselves, or their past. Some strength, weakness, and so on. Their experiences make them wise, and it adds a very real, very down-to-earth feel to an RP.
For example, my longest-running character, Allegra, is now a 6-year-old red wolf. She was 3 years old when I first started RPing her, and since then she's been through more experiences than most can say they've even written out before. So yes, she is older when compared to the others in her pack, and she's gaining some white hairs where she didn't have them before, she has old wounds nagging her more than they used to, but her sheer experience as an Alphess, guardian, and whatever else she's needed to be for her pack makes her a very well-developed and colorful character that I adore playing. In most cases, she's so well-developed that I don't even have to think about how she'll react in a post-- she seems to just react on her own in my head, and all I have to do is describe it in my post.
All in all, Allegra is older, but she's still oodles of experience under her belt, with a few good years left in her physical body.
This said, I wouldn't even consider a character legitimately 'old' until they're reaching around 9 or 10+ years, depending on the species.
That's why I always smile when I see an older character in-game, since I know there's almost always a little more than usual under their white-muzzled surface.