How do we know when a person in a weaker position can consent to something of their own free will without being manipulated to an unacceptable degree?

We have ages of consent in the US. It varies by state. There are different ages at which you can agree to marry without your parent’s approval, and different ages below which any sex you have with an older person is statutory rape.

Throughout history, older men have hit on younger women for sex. They might think it is easier because younger women may have less experience and fewer defenses than older women. Many would say such men are preying on the young women.

Of course, sexual abuse is an extreme example of this behavior; an example that is almost universally considered something that no one should do. It should be punished, most agree, although many societies put such laws on the books but never enforce them.

So what qualities does a person need to have to be able to control their own life and consent to decisions like having sex or getting married or even being considered a fully moral agent who can be prosecuted as an adult in court cases? Please bring in examples, as that is easier to understand, but feel free to attempt to make sweeping generalizations, too.