I may get banned for even posting.Some on the forum get a little twitchy when it is suggested people should marry younger than 18 even when that's quite legal in some countries. I personally know of people who married at 16 and who have remained married to this day.
My wife and I got married at 18 and are still married 42 years later.
Some states allow you to marry at 16 with your parents permission.
As for David I agree that he sinned first when he covted his neighbor's wife (after he found out that she was married) and again when he committed adultery with her.