There is no age limit. The Nintendo DS (and 3DS) library spans E (Everyone) to M (Mature) ratings, appealing to kids, teens, adults, and seniors alike.
Take breaks every 30–60 minutes (20-20-20 rule: look 20 feet away for 20 seconds every 20 minutes). Small screens at close distance can cause eye strain.
Play as long as it brings you joy. Shigeru Miyamoto (70+) still plays games. No "too old" exists.