(For the most part)
I've come across a lot of people in my life, judging me by my age. I can't count the number of times I have heard "(...) but you wouldn't understand, you're only (age) years old". Some of the times it has been the absolute truth, but most of the times, it was used as an excuse to say "I do not have any more points in this discussion, so I'll assert dominance by talking down to you".
Judging people by their age, has suddenly become a way of "winning" arguments, instead of actual factual statements. A persons MENTAL age is, even though a lot of people don't care to admit it, only partially decided by the number of years they've lived.
A person can experience 30 years of happenings in just 15 years, and the other way around. Mental age is determined by the environment you were raised, the people you spent time with, the stuff that happened around you, and a great number of other things.
Talking down to someone with the intend of saying "hey, listen to me, because I'm older than you, and my statements are therefore more reasonable" does not make you mature, so why do people still resort to it?