I've been the target of this. To this day, and I'm nearly 29 years old, I still say "ma'am" to women and "sir" to men.
In some ways I appreciate it, because it has made me quite a respectful person. It depresses me when I see children being egotistical little snots, saying some of the dumbest stuff expecting everything.
But the real thing about ageism is that children are considered not "as capable" as adults. They cannot "make decisions" as adults can (somehow, magically, the day between 17 years and 365 days and one day later equates a significant leap in development).
My ex has remarked on several ocassions about how I treat her younger sister and her friend, as if they're capable human beings, "You don't talk to them like kids, it's really sweet." Adults talk down to children. Water things down and make children out to be somewhat less somehow. God, baby talk, just imagine. I've read that baby talk burdens a childs development because it is as if you're trying to teach it two languages at once. The parent talks to the baby in a watered down language, but to other adults in regular language. So the baby has to figure out context, which takes a little longer.
There are lots of scenarios where children are considered "less" than adults. It's a difficult problem.
I am a leader, but you will not follow me.