On his blog, the show’s creator Tom Ruegger shows a scene that got missed out of the original show as it was considered inappropriate. It does however prove that there’s no way Wakko was intended to be “only seven” as the song might claim.
It’s a running joke in the show that the Animaniacs are of an indeterminate species. When asked, they will avoid the question and simply say, “We’re cute.” One of the show’s other writers is quoted as saying that their species is ‘Toon’ or Cartoonus Characterus (a species brought to attention in the film ‘Who Framed Roger Rabbit’).
This claim is backed up by the short ‘Draculee, Draculaa’ in which Yakko says that, since he and his siblings were drawn, their parents must be pencils.
So it would follow that if dogs can be 7 in human years but equate to an actual age of 44-50 in dog years, Toon years might be different from human years too.
Toons never really appear to change from episode to episode, implying that they do not follow the standard rules of aging. Truth is, they don’t.
Cartoons are like the wizard’s photos from Harry Potter, they only capture the characters at a moment in time that is frozen forever. Toons will only age during the time they spend off the screen. This is why most characters will appear different if their show has a break and isn’t written or shown for a while. Of course this means that it’s impossible to accurately tell how old any given Toon is. In the case of the Animaniacs, though, we do know that they were locked in the Warner Brothers water tower between the 1930s to 1990s, making them each around 60-something.
This would seem to be an accurate guesstimate since Wakko is clearly a dirty old man.