We did originally think we would cut down on presents after our dc reached 18, but then realised - why should we? We give our kids presents because we like giving them presents, not because they are a certain age.
Yes, adults are tricky to buy for, because after a certain point they buy for themselves. So we all post wishlists on our family WhatsApp group, and it's up to everyone to choose whether to buy from that last or to buy something they fancy giving.
This year we knew our eldest wanted a good woollen coat, so we said we'd get him that without him asking. He has also posted a wishlist - but it's all small things, so he's clearly taken our offer into account, and put up little things for his siblings to buy him.
Sometimes items are very specific, eg if there's a particular widget one of our tech-heads wants. Sometimes they're not, eg dd wants a laptop case, so dh and I had fun creating a custom one with images of her pet.
TLDR: spend what is comfortable for us, gives them things they need or will enjoy, and gives us pleasure. Ages do not come into it.