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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.
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.