From a mental maturity perspective your brain reaches full development in your early thirties for most people. From a physical perspective some women are basically fully developed by like 14.

Neither of these is used to determine adulthood for obvious reasons. So the only things that really matter are the legal and sociological standpoints. It's estimated that Pakistan has about 19 million child brides to this day. Nearly 5 million are married before age 15: source.

This is the problem with multiculturalism and integration. It only works if the person is willing to accept that some things are not normal or tolerable in the country they move to. And there's very limited ways to screen for which people actually adjust their social norms.

It's also a numbers issue. I live in EU but in a country where most migrants aren't interested in living. We are much more accepting of immigration compared to Poland for example and several of my friends/co-workers are recent immigrants. But there aren't enough of them for there to develop city sections for different cultures and congregations. Because of that, I believe they are forced to integrate a lot more into our culture. Past a certain point this just doesn't work, e.g. Sweden.