My guess is that Scamp is under a year old and Angel is older, especially considering how many families she's been through and her experience on the streets and with the Junkyard dogs. She must've been with them for a while. My guess for Scamp would be 7-8 months and Angel about a year, year and a half.

I am not sure they ever actually gave her physical age in relation to Scamp. It is, however, implied that Angel and Scamp are, essentially, the same age. Technically, if the sequel occurs during the Fourth of July celebrations in the town, or just after, then that makes them about seven months old...give or take. (Which is why I find the use of their voice actors rather out of place...as the voices are far too old in sound for their apparent ages).