Op, we have the opposite problem.
We have 2 sets of friends (same friendship group) who we exchange birthday and Christmas presents with, both with 2 DDs. I have DS and DD. Ours and family 1 DDs are same ages (20/17), Family 2 Dds pre/young teens.
Eldest in all 3 families have birthdays with 6 days of each other in Nov.
Youngest in other 2 families have birthdays on almost consecutive days in October. My DD birthday is in Early June.
So out of our 6 DC, DD is only birthday not in October or November.
They almost always forget DDs birthday. She usually gets something, but weeks late and the one with younger girls then sends random tat. One year it looked like she had just picked random bits up in her house including small packs of sweets.
I did hope we could stop at 18 but when the 2 eldest (incl. DS) turned 18 two years ago, the 2 friends were both keen to carry on.
It annoys me, but as DH said, the friend with younger kids is lovely, just disorganised and I do value her friendship, so not worth falling out over.
Christmas is generally fine, usually a gift card or money, so I know DDs birthday is just forgetfulness/disorganisation.
Others who suddenly stop, then I do just stop theirs but would prefer it to be agreed, in case something has been posted and just not arrived. DH uncle has one DD slightly older than my kids, so although she is DH first cousin, she is only slightly older than my 2 and their cousins. Grown up together, always bought at birthdays and xmas.
I have carried on buying for her even past graduation as she has only PT work and not settled into a career yet. I know my SIL was planning to also continue presents till settled in a career.
This year, DS didn't even get a card at birthday (but did get a FBook msg from DH uncle) and we only had a card at xmas. I have no problem stopping presents, but would prefer it to be agreed.