When I was a kid it just died a mysterious death sometime around the age of 18-20. I think it either gets to the point where they become and adult earning their own money and they reciprocate and it continues or it stops.

When it's everyone getting together and there is a bunch of teenagers around the 12-17 mark in your case it may be tricky though because if you cut it out at a certain age then the older ones would be sitting there with nothing while watching the younger ones open their presents. So I would say either wait until the youngest of that group reaches a certain age or another option is to introduce a secret santa/kris kringle kind of thing for extended family?