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.
I don't think I have advised anyone to wear a helmet, but I might point out that it was pointless wearing one if they weren't going to do the strap up, or it really wasn't adjusted properly to their head. I have been known to adjust friends helmets - but tell them to wear one - NO. (Well excluding my kids when they were younger, now in their teens they can make their own mind up.)
Equally I wouldn't tell off someone for not wearing a helmet either.
I prefer to quote statistics and papers at those who have the temerity to question me on my (lack of) head wear. It won't change their minds, but at least they'll learn that asking me silly questions leads to painful and unpleasant experiences. Think of it as a form of aversion therapy.
I generally prefer to believe that people are able to think for themselves (however contrary the evidence sometimes), whether it involves helmets, P&L, fairly trivial things right up to learning high level maths. It's not anyone's job to do their thinking for them and that principal has generally led to good reception on a wide range of topics.