Well, no, that's not actually true.
Anyone who gets married before they become an adult scares me a little.
After all, it doesn't really have anything to do with physical age, does it? It's a matter of how mature, how responsible, and how well-prepared to support a family they are. And that can happen at eighteen, or twenty-one, or twenty-five or forty or perhaps never. It all depends on the individual, and how willing they are to take on the responsibility of a family.
Age is a trade-off. Younger husbands, wives, and parents may be less mature, less experienced, less wealthy, or less responsible. But they're also likely to be more energetic, more ambitious, more hopeful, and more able to relate to their children. Older couples may know more about how to make a relationship last, but younger ones will have less baggage from previous relationships. Older spouses may have more financial resources, but younger ones have more flexibility to grow their careers. And so it goes.
Commitment has nothing to do with age. You don't need to play the dating game for ten full years to know what you do and don't want out of a lifemate, or even whether you're the type of person to be one. You don't need to date or live with someone for three years to know if it's meant to be. A middle-aged adult can be just as faithless and uncommiting as a teenager with raging hormones. It all depends on the individual.
I would suspect that the problem people have with young married adults, then, is a subjective one. "These two are as young as me or younger! And I'm still broke and hedonistic! How can they be any different than me? How can they take on the kind of responsibility that should be reserved for people who look like my parents?" Guess what? Maybe they can. And maybe, by the time you're as old as your parents are now, you'll be just as broke and hedonistic as you are today.
Don't judge others' ability to found a family based on their age. Besides, it's fairly common psychology that the best way to improve someone's sense of responsibility is to give them some.