They were both Aurors so they have to be at least 21 (Auror training is three years). They could, theoretically, be the same school year as Lily and James but I doubt it. For one thing Alice could not have been a fully trained Auror if she had to take time off to have Neville (he was born in 1980). So I think her and Frank have to be at least a year older than James and Lily - but probably more.
Were they the same age as Lily and James or older/younger?
I agree with Merlin_Helz, but again, it's never stated so you have some wriggle room.