He was upset because society uses age as a stereotype in the form of legal ages to do things. Why do we use age as a stereotype? Because it's a good measure of responsibility in youth. Twenty-one year olds are going to be more responsible than eleven year olds when it comes to alcohol, etc. It's ridiculous to not be able to make generalizations about the majority of a group because not every single member of the group is like that.
Stereotyping about age is very different then stereotyping about race. There are huge differences between an 11 year-old and a 21 year-old, for one, an 11 year old is much less mature and experienced then a 21 year old, that is why a 21 year old can drive, drink alcohol, etc. But what is the difference between different races? The only thing different is the way they look. So how can you form a stereotype just by the way people look? So your age stereotype analogy doesnt work very well.
I wasn't suggesting that there were any differences between the races. I was simply pointing out a case in which stereotyping works for a purpose. Stereotyping based on race doesn't necessarily suggest that one group is more adept or anything. Such as with my social example, stereotyping allows me to avoid potentially uncomfortable situations.