DH has always said once they get to 18, we'd stop buying (we only have nieces/nephews on his side). I've always said if we still clearly have a relationship into old age, I'd like to give them a little something.
So at 18, we stopped.
N/N 1 - we haven't had contact for 18 months or a card (gave one last year), but were invited to a special birthday at that time. Lives locally.
NN 2 - only see if families are getting together and she's at home with her parents - lives locally. Has given us an Xmas present this year via parents. My godchild, who is absolutely lovely
NN 3 - never see or hear from. Parent is clearly still upset we're not buying for.
NN 4 - lives away, comes back occasionally. Asked to see us last year and gave us a present. Asked to see us again tomorrow.
I'd really live to give NN4 a little something, but is related to 1 and 2, who will see over Xmas. DH says 'no'. If not, I've suggested we buy 4 a drink when they come to us (pub very close), and suggest meeting up with 2 for coffee which they like.
What's fair?
A sideline, BIL always said he'd stop at 18, but now gives to our DD as he sees her 2/3x a year, she'll babysit/take his children out then. It's clear for him as he has no contact whatsoever with rest of N&N