I think it sounds like you sort of want to stop. Is that a good guess?
Stopping at a certain age seems perfectly fine. It does seem fairer, to me, to do it that way. Then the kids who are younger don't lose out. and if you are ever challenged (which would be rude beyond words, btw), you can say, "Oh, I decided to stop when you get to be 25." or whatever.
And I do think that once they get to be grownups, you SHOULD perhaps stop giving presents to people who don't *ever* give you a present in return.
And once gift-giving devolves to an obligatory check, I personally think it should probably stop. It's one thing to say, "you're going away to college, here's graduation money to help you with the transition," but I sort of think that if you don't know someone well enough to pick out a present, maybe you don't know them well enough to buy a present.
In other words, maybe your niece and nephew have the right idea on whom to give presents TO--people you are really, really close to.
I'm also a fan of the idea that one year you might give a b'day present, and another year you might not. I reject the idea that among grownups, it must be "fair." (though I admit to sometimes being comfortable with that, personally)