I don't know. While my natural inclination is to agree with you Cogito for exactly the same reasons (changed a lot in my 20s and outgrew my XH), I have noticed that the couples I know who are celebrating long happy marriages all got together in their late teens/early 20s. That would include my own parents.
I think that if you marry young, you either grow together or you grow apart. I also think you can influence which of those outcomes occurs. To do so requires a certain level of self-knowledge and maturity in both partners, but those things do not necessarily coincide with biological age even though it is more likely.
My own observations on life have led me to conclude that there are two types of marriage. There are those whose lives are defined by their marriage. They often married young and they build a life together. That's all they know and they are content with it. They are a unit.
Then there are marriages between people who have established their own lives and are fiercely protective of their identities. They are much pickier about who they will share that with and what compromises they will make. Assuming no major 'issues' this tends to make for a marriage with a lot of healthy boundaries and respect. It is an equal partnership between two distinct individuals. I see this in a lot of second marriages (the sort where people have learned from the first 'mistake' rather than being doomed to repeat it) or older marriages.
Between those two extremes, I seem to see a lot of conflict between what people want as individuals and what they desire as a relationship. In particular I see a lot of people in their late 20s/early 30s who have come out of one relationship and are experiencing a lot of dissatisfaction with their lives (maybe career has failed to take off or they have money worries, etc) and expect to find a solution and fulfilment through their relationship. Those sorts of marriages seem to fail quite a lot IME.
Ultimately, though, maturity can only take you so far. We all know people in their 50s who have the emotional depth of a 15-year-old when it comes to being in a relationship.