Comment by Galactic Navy Officer on Chris

@Chris Well, let's guess he was nearly two years old when the sister was born. She's nearly five, I think? So at six, he wouldn't be elderly, especially since he's a small dog and apparently a mutt, but he'd be headed for the human equivalent of forty.

I'm guessing at least one year old when MC regressed, since the dog hasn't grown any further since MC was reunited with him.