How old is Android 18 in the cell?
Assuming their number designation could be a hint to their approximate age when they were introduced, as they were kidnapped as children, this would make both Android 17 and 18 in their late 20’s or early 30’s by the end of the Dragon Ball Super anime.
Can Android 18 get old?
According to Toriyama in an interview, yes they age. However they age slower than everyone else. Probably even slower than the Saiyans. I don’t know if he touched upon whether or not 17 and 18 would outlive most of the other cast.
