I honestly never understood that whole age is just a number phrase, becuase age does matter in some instances. I'm 30 years old. Wouldn't people have a problem with me dating somebody who is 15 (not that I would ever since I'm attracted to people one to three years older than me). The kid is not even fully developed, mature, and could easily be manipulated and taken advantaged of.
I say the point where it kind of does NOT matter is the moment where both are adults. So, if a 20 year old wants to date somebody who is 50, so be it. The thing to consider though is that the two people are on totally different life experiences and stages. The 20 year old could be full of energy and into certain things, whereas the 50 year old just want to settle down, take it easy, and may have old fashion views than the person he/she is dating.
18 years is considered an adult by the state, and in this case, who really knows if at this age they are mature and know what they want. But it's up to them.
Nope, it's definitely not just a number. Only people who say this don't really think a lot about anything - they just dive in head first and usually get their skulls broken... Or they're people with their own agendas which benefit only them.