Message to folks in their 20's: You know a whole lot more now than you will know in your 30's. (that is to say, conversely, you will know far less in your 30's than you know in your 20's)
Message to folks in their 40's: Don't tell me what I'll be missing by not reaching my 40's. I don't want to know.

And still, it's funny, turning 30 is seen as this huge leap in age and you're really, really old. That's how I always saw it. It's not. It's only a huge leap in responsibility (which happens at different ages for different people anyway. the 30 benchmark is really just symbolic.). I, myself, am declining the responsibility, but in terms of maturity, experience, and confidence, 30's are halla better than 20's.
It's just that all the stuff you knew in your 20's and were so certain of, you find you don't really know and you're not so certain.