Adult stupidity

As a kid it's easy to criticise adults for acting slow, having less wit, being more critical and judgemental of everything, and generally "not getting it."  However, being an adult, I wonder if stupidity really does set it, causing one to think slower and "get it" less as early as 20-30 years old.  Maybe it's just a downward spiral from when you think you know everything to when you become senile.

What the fuck is this shit? You don't stop learning at age 21. Your brain might be fully developed at that age, but you sure as hell don't stop learning, and nothing keeps you from learning either. If you're a stupid adult, you're just a stupid adult. Not much you can do about that.