I'm not sure about the aging issue of the sim and son (I uninstalled ITF until I finish my 4x4 Dynasty).
As for the pet, it's a known glitch. People (including me) have even adopted kittens/puppies/foals that arrive as adults, and same day, age to elder. Something about the coding that says, "time for a birthday!" and so they age up on the way over.
In your case, if the dog was showing as an adult when you adopted, it's possible that it was simply that dog's time to age up.