I don't know about normal, but it seems a little odd to me. It should be you giving the back rub and not some kid. Time to tell him to hit the road and do it yourself.
Is it normal if my wife (50) gets a back rub from an 18 year old, when me (her husband) is in bed with very little on?
This actually does seem quite strange. Is that 18 year old your child? That would at least be a bit more understandable, but if it's not......
If you have no affiliations with that 18 year old, tell him to back off. And if he starts getting violent, call the police. He could be charged with assault.