The girls you met very well may be not mature to your liking, but that's how women are. They're generally less mature than men; I'd wager that there's about a five year window where a man can be younger than a woman (speaking generally, of men and women "at random") and as or more mature than her. That's part of being a husband, i.e., being a moral leader and setting an example of virtue and behavior to be emblemized by your family.
Keep in mind that people change. I'm not saying that you shouldn't be sensitive to marrying someone who's immature, I'm saying that some discrepancy in maturity just comes with the territory-- the territory not so much of age, but of men and women. The degree to which a girl is less mature than you is a degree to which she can improve. You're going to marry someone and be with them for the rest of your life, so while you can of course only judge what is known (i.e., her personality now), you'll want to keep in mind her capacity to develop. To that end, I'd say that a tendency toward wanting to be better (in any/everything) is more important than present maturity levels. If you met a girl as mature as you who also thought she had everything figured out and didn't need any direction, that'd be a far inferior match than a girl who is less mature than you but who has a genuine passion and interest in improving herself.
Maturity is highly influenced by environment, too. If you find a girl your age who was raised in an environment where she had a lot of responsibilities-- say, she was an older sister, or grew up on a farm-- I think you'll find that those types of environments tend toward a more accelerated acquisition of maturity.
And finally, don't confuse a high intellectual capacity and tendency for maturity. You may actually be finding not that the girls you are meeting are too immature for you, but that they're just not as intellectually bent. Remember that women are interested in people, men in things. And that's good, because it provides a balance for the home life. You're only "partners" in the loose sense of the word; really you'll be husband and wife, and you'll each have discrete and distinct duties that focus on cultivating different aspects of the home in order to make a "complete" healthy, Catholic environment for your children. Don't turn down a wife because she isn't as "scholarly" or "intellectual" as you are.