It's arguable that as long as you are mature enough to be able to compromise, hear another person's perspective, and work together, there is limited maturity necessary for marriage. One doesn't need to be a finished product, and quite possibly being a finished product and completely mature is counterproductive to growing and molding together. Someone who has been living on their own completely autonomously for a decade will have a significantly more difficult time adjusting to marriage than 2 young impressionable malleable kids.
As someone who lived on his own for almost 10 years before getting married, I don't necessarily agree with that, and definitely not as a blanket statement. It's very much dependent on the individual.
While individual circumstances might be different, I strongly believe (and my mechutenste who has a wedding gown business, and gets to see many kallahs, concurs) that it is much healthier and easier to get married young and to mature together as a couple. The older people get, and the more their own habits and ideas get ingrained in them, the harder it might be to build a harmonious home.