Until my children were twelve I liked them to wear one. I was advised by friends with a medical bachground that after twelve-ish the skull is tougher and better at bouncing.
During that time I often wore one to set an example. Now they are all 13-plus and they make their own decisions. One of three still wears a helmet.
I'm not sure why I'd ever advise someone to wear a helmet. I don't see them as dangerous, but I don't see them as particularly important either.
I do advise people to wear gloves and eye protection. I've had (and witnessed) plenty of issues with gravel rash on palms and nasty flying things or rain, hail, sleet, stone chipe pinging into or around the eyes.
All my kids wear gloves and glasses when riding.