We've continued until age 21.

When they reached 18, did you just stop giving gifts? Or did the parents say something?

I have always given gifts to my sister's daughter who is 11 now. Both for birthdays and holidays and sometimes even random gifts if I see something that I know that she will like.

For my cousin's kids, we usually do birthday gifts and we bring gifts when we visit.

Yes. My one BIL and his wife have 4 kids. We have 2. We've always sent back and forth to the kids, but I must say it is getting expensive now that there are 4 of them.

Yes. I do a toy for their birthday and an experience gift for Christmas - trampoline place, movie tickets, rock wall climbing, etc.

We did until all the kids turned 18. There are a large number of nieces and nephews. Now I just wish them happy birthday with a text or on FB.

I'm an only and DH just has his brother. We buy BILs 3 kids birthday and Christmas gifts every year. We used to try to call or FaceTime on birthdays as well, but eventually stopped as it always felt like we were interrupting something. I grew up close to my aunts and uncles, so it just seemed like the thing to do.
They do reciprocate with our kids.
If we had a ton more neices and nephews, we'd probably spend less.

I did until I realized how much the children in our family (including my own) receive in the way of money, gift cards and presents. If we are invited to a party, of course we bring something but otherwise, it seems wasteful. I think the idea of contributing to a 529 is good.

I have 11 nieces & nephews, and started sending small gifts for all of them a few years ago. I budget $25, and send gifts/books for the younger kids, amazon gift cards for the older kids. Some families reciprocate but most do not. I decided not to care about that a long time ago - I had that one aunt growing up that always remembered my birthday with a small cash gift and it always felt amazing. She never had kids, and now continues the tradition by sending my kids the same small cash gift on their birthdays & major holidays. I have a very large & mostly long distance family, so I appreciate her way (and now my way) of maintaining a connection with everyone.

I think it's important to take a look at the ways we pressure ourselves, but attribute to the pressure to others. There's no reason why a gift has to be a certain $value, other than what your budget can afford, or a certain number of items. There's no reason to think that gifts have to be reciprocated in kind or kept equal. Gifts should be freely given and accepted in the same spirit. Try to clear out all the pollution, and look simply at (1) what do I want to give. Do that. The rest is clutter. People will think what they want, project what they want, be offended however they want.

Only one niece I spent about $100 each on birthday and Christmas
She's 22 now so it's usually a gift card or cash unless I see something I know she will love

We do, and most of our siblings reciprocate. Only one recently stopped buying for ours but I still buy for theirs. It's not the kids' fault, and I'm not about to buy everyone else a gift and not them.