Well, for avoiding fornication 15 to 18 would be prime years when the urges kick in. Do you have any stats collected from first century Greece to back that up.
Statistically, though, marrying young seems to correlate with high divorce rates from what I've read. But regular church attendance correlates with lower divorce rates, and female virginity at marriage (old research) or else only having fornicated with the man she would marry correlate with lower rates of 'marital disruption.' I'd like to see if those who are serious about their faith who marry rather young have relatively low divorce rates.
It's also hard to earn a living at 15 to 18 years old. Now that we are going into a demographic desert with a smaller working generation, it might become easier to do so without a college degree.