The trade-off was that they wanted Alan Rickman for Snape. So that meant Lily and James needed to be older too. Since we only see them in a few flashbacks I don’t think it’s that big of a deal that they’re older than written.
Yeah, Alan Rickman was too old to protray Snape. Not his fault, and he did a fantastic job with the character, but it would be interesting to see versions of the characters with more accurate ages.
It also better explains the vast fortune they left Harry without having to bring in all that bullshit from pottermore. They DINKed it up for a decade and a half before popping one out.