How old are the পরবর্তি generation HP?

Teddy would be 19, since the epilogue is set 19 years later from the main book, which is when he was born.

Victoire would be 18, because she's born on the anniversary of the Battle of Hogwarts (that explains her name - it means Victory)

Both Hugo and Albus would be 11, in their first বছর of Hogwarts.

Those are the only পরবর্তি generation characters whose ages are precisely known, but we can guess estimates for some others:
I think that James is 12 অথবা 13, সেকেন্ড অথবা third year.
I think that Rose and Lily are about 9 অথবা 10.
We don't really know for Scorpius, sorry. I'd think he's between 11 and 13 though.