"Age discrimination" is built right into the U.S. constitution. Age 35 to be President, 30 to be a Senator, 25 for Congress. So the Founders obviously had in mind a sliding scale relating age to rights and responsibilities.
It's true that some people under 21 have more wisdom and maturity than some people over 65. But, practically speaking, we can't make rules based on the outliers. We have to deal with the averages.