Why do we need to "grow up"?

How much does one person's personality and attitude need to change as they grow older? I feel like most people around me haven't really grown up. We're mid-to-late 20s, even early 30s, and we're all just as giddy and immature as when we were young and stupid. It's fine by me. Growing old doesn't mean acting old.

Being young at heart has nothing to do with being responsible. We all "grow" as we do more in life, and having kids will probably make one think that s/he needs to "act mature". Maturity is subjective, and really doesn't need to be applied in certain aspects of life. Do maturity and responsibility go hand-in-hand? I'm not so sure. I know someone running a multi-million dollar company who still acts pretty immature at times. Trying to be mature all the time is not only draining, but it's boring. I'd advise against it.

Just rambling...