I think one of the biggest disappointments in reaching adulthood is realizing that getting older does not give a person some special wisdom. For example, when I was a kid, I always thought those old guys smoking cigars at the bowling alley were pretty cool. Now I realize they were just... old guys at the bowling alley smoking cigars. Men who may or may not have had family problems, substance problems, etc.
Unfortunately, neither getting older, getting married, or even having kids automatically injects wisdom into a person. But I guess if adults told kids that, then it would probably create some problems in the social order. Instead, kids are told about how great things will be when they grow up.
If anything, I see people getting worse as they get older, not better. Less creative, less flexible, and less athletic. There are exceptions of course, and everyone would like to be one of these exceptions, but in general I see a trend for people to get worse.
Except for rational thinking, it seems like almost all other qualities are amplified in youth and then gradually fade away during aging. People usually become better at solving day-to-day problems as they age, but this is more due to experience than anything else. In evolutionary terms, the mating sweepstakes is in the younger years, so it makes sense that this is when certain qualities would be at their peak.