If we assume that most people actually follow rules and never break any laws, the following things would happen:
+ no children get pregnant before 18
+ more attention for other things, like school, thus
+ more people get a successful career
+ side-effects of break-ups, like shootings, violence against parents, friends etc. will decrease dramatically
Unfortunately, this is based on emotionally stable persons (teenagers), which is nearly impossible, who also never break any laws, a very rare occurance.
Instead you'll see something like this:
+ less children get pregnant before 18
+ attention for other things, like school, won't change much
+ some teenagers are emotionally so unstable that they'll ignore this law, get it on with a girl/guy and start a family at the age of 15/16/17, thus
+ those will totally fail in society
+ side-effects of break-ups, like shootings, violence against parents, friends etc. won't happen as often, but when it happens, it will be much more violent
Problem with a law like this is that it goes right in against the 'goal' of every living being: pass on its genes, so you'll get:
'intelligence'/reason <--> primal instincts
Since by most people, and especially teenagers, the 'primal' side is much stronger, this side will triomf over reason by those.
On those who were already emotionally stable, this law won't have much effect, but on the less stable persons, this will have a diastrous effect. It'll cause them to become outlaws because they can't control their instincts.
Conclusion: this law will have more negative than positive effects, and is therefore not advisable.