Well Rome is difficult, what you said is more true for the Republican era, after that Rome didn’t need to “produce” soldiers they simply offered Roman citizenship and benefits. They were largely able to reintroduce recruit from Provences. The promise of good pay, war spoils, and land also helped.

As for marrigeable ages, your sources mostly pull from written documents. Roman society was aggressively patriarchal so daughters were often used as a way to broker alliances or an increase in social status. There is also the payment of dowry that incentives men to marry off their daughters as soon as possible