"I and most other people"
You and most people 3000 years later in a culture that offers thousands of other options versus the scant few they had.
"Because children shouldn't be married off to adults"
They define adults differently. Old enough to work, old enough to reproduce...then have at it. You're an adult. That applied to men and women. It was any "able-bodied" person that went to war when the call came. What else were they gonna do...play SEGA while they wait on some arbitrary birthday to arrive? Teenagers in Middle school are giving birth all the time, showing that the human body is mature enough to handle the task. The only thing that makes us wait is the fact that we don't like these unprepared kids dumping their parenting obligations on the rest of society. The moral argument is based on Judeo-Christian beliefs and sounds silly coming from atheists. It's based on a universal standard for all. Atheists cannot apply a moral argument on anyone other than themselves, so judging, even the guy next door, is antagonistic to an atheist's worldview and a glaring contradiction to how they want to be treated by others. For you to say something is wrong...for you, but somehow stand by the statement that it is just wrong while taking a position on a sub-culture that lived thousands of years ago is recognizing the dilemma, while embracing the contradiction you've created.
Good question: Are we allowed to criticize ancient cultural norms that are different today?
Sure, but with an understanding that we aren't in their shoes, but are a culture that owes its existence to them and their ways, a culture that has thousands of years of education and history to learn from, whereas they had, at best, a solid family history and local group of leaders privy to a little history...that looked much like their current situation since knowledge was usually localized.
Would it work in their given situation, or would it hurt?
Would an age restriction on marriage make them stronger or weaker compared to surrounding cultures, with their own beliefs? It would definitely hurt their ability to grow, which was crucial to the survival of a culture in competition with surrounding ones.