Who said a person is sexually mature because his sexual organs are capable of intercourse ? Sex is at least 50% an activity of the mind. A sexual relationship, to be healthy and beneficial to the person, requires emotional and intelectual maturity. Nobody in this world has ever been mature for sex at 13. Just see the destruction that sex among teenagers is producing right now.
I will not even try going into the issue of sex having spiritual implications.
You just did use your modern spiritual beliefs on that.
Read up on the ages of historical figures all the way into the first part of the 20th century for the west, and look at non western nations then, and now for some much needed clarity.