That is such an arrogant statement. 21 year olds are citizens, and their beliefs and thoughts matter just as much as yours. They may not have a full appreciation of how taxation effects their lives, but you may not have a full appreciation of how important things like education and social welfare are to society.
It's so typical of the baby boom generation at this point. "Oh, sure... I got all these benefits that my parents fought tirelessly for, but I mean... you young kids don't deserve the same benefits we got. You're just entitled, and ruining my ability to retire comfortably!" Absolutely selfish.
Let's just take away any age restriction then. Why not let 12 year olds vote? They're citizens, their beliefs and thoughts matter just as much as yours. You don't know their life experiences, heck they might be smarter than any of us. Sure they might not be able to drive, work, have consensual sex or get into R-rated movies by themselves but their opinions are no less valid than yours.