Actually, the latest medical studies on the human brain suggest that full reasoning capacity isn't actually reached until something more like 23.  I think the only reason voting age is down to 18 is because the political reality being that the military needs/wants people in the 18-21 age bracket, and they therefore deserve some voice in the political process. 

In the 1960's the conventional wisdom may have been "don't trust anyone over 30."  I'm only 37, but I'd like to submit "don't trust anyone under 25."  Seeing any college campus on Thursday through Sunday nights ought to make that fairly obvious.