Re: If your son who is over 18 asks you how to bet on soccer, would you help him?
Yes I would teach him about responsible betting. As of how to bet on soccer, that's not something you can teach anyone about. I'd suggest him to Google it and I'm pretty sure the result would be better than me teaching him. Btw, in my opinion 18 is too early to start betting. I'd suggest him to wait until he's 21 at least. Also I'd tell him that those who say they can teach him how to bet on soccer and make money from that are just charlatans.
Google has provided many ways to choose betting options in football, whatever you are looking for will be there. 
Yes, regarding age, the ideal age should be 21 years old because 18 years old are usually still studying at school, so they should not be addicted to gambling before finishing their school.
Never promise anything about gambling, there is no better result, but there are more losses.