My DH has 5 nieces and nephews aging from 28 down to 19.
We stopped putting a cheque in for the 28yo a couple of years ago, when it became apparent she earned more than both of us put together.
The next is aged 23, and she got £25 last year. Her sister is 19. I am of the opinion you can't send a cheque to one sister and not the other, but the 23yo is working, and lives with her parents, so has few outgoings. She is in the process of buying a BMW.
The others are aged 20 and 22, but are still students, so we would send them something.
Just wondering when you stop sending money. We haven't actually seen them at Christmas for about 8 years, so are unlikely to buy them actual presents.