Well, I have several grandchildren. For many years I have given them money at Christmas and for birthdays. Every child is different. One texts me thank you, one messages me on Facebook, one I don't hear from. I love my dil and son dearly. They are terrific parents, but thank you notes are just not on with them. As the parents go, so too the children.
I don't give gifts to get thank yous, though they are nice. Money is tight there, and I know the extra helps. My son doesn't ask me for anything, ever, so I'm good.
I do have a cut off age for them: 21. Seems reasonable.