I think its a little bit unreasonable to stop at that age, just based on not knowing what to put in them! I still do them for DS19 & DD16.5. They love them, DH and I also do one for each other.
Generally they get socks, pants, smellies of all kinds, chocs & sweets, revision items (cards for doing notes, highlighters, coloured pens etc), toothbrush, toothpaste. Most of which you can buy in a supermarket/online. Add a satsuma. Some mini pringles or such.
You dont say whether they are boys or girls, but other ideas are a book, DVD, phone case, charging leads (always breaking in this house!), standalone chargers, makeup/jewellry for a girl. Hot choc items if they interested in that. Calendar if they'd use one, though I have decided against that this year. DS now at uni so he getting some food items and alcohol.
There are a number of threads on here with great ideas, one was a wind-up toy each that they can use for races on xmas morning!