If you stop buying for nieces and nephews once they turn 18, but your children are still say only 10... you can guarantee your siblings won’t be buying them presents for the next 8 years!
Why not *QuickBugQn ? 
My dc are the eldest by some way on dh's side (my dc1 is 22, youngest nephew is 4). I'm the one who instigated the 'don't buy once they've turned 21' rule, and we still buy for all my nieces and nephews on dh's side. On my side, my dc3 is the youngest of all the cousins, and my siblings still get for her (and dc2)
I would never stop. My parents have never stopped giving me Christmas presents and I would never stop giving to others or my daughter. Why does age matter? It's just thoughtful to give gifts at Christmas isn't it?
FllofenHoofen - people aren't talking about their own dc, they are talking about nieces and nephews.
As adults, they then start getting partners..... at what point does 'the boyfriend' get a gift too ? - after going out a certain number of months of year, or if they live together, or only if they choose to get married?..... what about when they then have dc, the numbers start getting ridiculous....... what about if their partners already have dc of their own ?......... One your nieces and nephews are adults, it is statistically likely you'll see them less often, so it is fr more difficult to know what they want / need. It just turns Autumn into a stressful round of shopping and more shopping and more shopping. 9 nieces and nephews (for us) was one thing, but start adding in partners you don't even know that well, and then the next generation and it becomes far to much to think about / keep up with - and even deliver or get to them.