These are not arbitary numbers, they are when most people end the growth stage of mental/physical development. This is why some countries have 12-14 (minimum puberty age) and some have 16-18 (maximum puberty age). I'd rather err on the side of caution and put it high to protect children as long as same age relationships are allowed within reason.
I think that the best way is for people to have starter relationships in the teens with people facing the same life choices and getting experience that way, not having some 50 year old guy come in with expectations of children, mortgages and dinner parties etc. There is no situation in which a 20+ year old should be with a 14- year old, ever. This is a social taboo anyway so I doubt it comes up much in Germany, people generally don't/don't want relationships with teenagers.
You guys should also look up "grooming" and how children are manipulated by abusers to think that they are making the choices. Even adults end up in abusive relationships, let's have some kind of limit to keep that shit away from still learning teenagers?
Are we getting at "Iain is 30 and would like to go out with his 12 year old next door neighbour" here or something? Do you think that is wrong Iain?