For the record, just because something is arbitrary doesn't mean it's bad. 
Just because something is random doesn't mean it's bad. 
I was once randomly assigned a phone number which was awesome. It was quick to say and easy to remember. 
In my country 17 year olds are allowed to join the army. That's an arbitrary age limit. It's oddly uncontroversial. That's because things are not bad because they're arbitrary. 
Things are bad, when they are harmful. 
This is a very important point. So I'm going to repeat it.  
*Things are bad, when they are harmful.* 
If someone has evidence that letting 17 year olds join the army is harmful, then they should present that evidence and we'll change the rule. 
Treating teenagers like children is bad because it's harmful. Demeaning, degrading, deriding, insulting, and infantilzing young adults (aka teens) is bad because it's harmful. We should stop that. 
This is not complicated.