Animals in The Wizarding World are much different from those of the "Muggle world"; pure speculation based on canon evidence give to us, but it seems reasonable to say that:
An Animagus' age isn't reflected in their animal form.
Consider this: James, Sirius and Peter were all around 15-16 when they first become Animagi.
[Remus] “Peter needed all the help he could get from James and Sirius. Finally, in our fifth year, they managed it. They could each turn into a different animal at will.”
At that age, you'd be considered an "adolescent". Realistically their Animagi forms would be of "adolescent" age as well. However, this isn't the case. Peter turns into a rat, too small for comparison. James and Sirius turn into a stag and a (rather large) dog, respectively. It's said (by Lupin himself) that 2 large animals were able to control Lupin in his fully-grown werewolf form.
[Remus] “Soon we were leaving the Shrieking Shack and roaming the school grounds and the village by night. Sirius and James transformed into such large animals, they were able to keep a werewolf in check.”
[Remus] “Well, of course, Snape tried it if he’d got as far as this house, he’d have met a fully grown werewolf but your father, who’d heard what Sirius had done, went after Snape and pulled him back, at great risk to his life, Snape glimpsed me, though, at the end of the tunnel.”
Or, more specifically: into such large animals. In other words, they changed into adult animals. I certainly don't see how an adolescent dog, or stag, could be considered as (such) large animals.
To reinforce this...
[Sirius] “And there was Peter, on the front page . . . on this boy’s shoulder. . . . I knew him at once . . . how many times had I seen him transform? And the caption said the boy would be going back to Hogwarts . . . to where Harry was. . . .”
Sirius hadn't seen Peter for 12 years since the incident1. If he was able to recognise Peter's rat form at age 15, to recognise him again so many years later would mean that Peter's rat form didn't technically "age", even though Peter himself did. Of course, this may all be referring to Peter's toe, but Sirius does recount the multiple times he had seen Peter transform, prior to the incident.
1 Peter "blowing himself up".
“Just before he transformed,” said Black. “When I cornered him, he yelled for the whole street to hear that I’d betrayed Lily and James. Then, before I could curse him, he blew apart the street with the wand behind his back, killed everyone within twenty feet of himself — and sped down into the sewer with the other rats. . . .”