The only thing that I am aware of concerning the use of the word teen is that in our advertising we have been instructed NOT to use the word Young preceding the word Teen.
There are good comments posted here but bottom line it sounds like Young Adult and Teen are considered the same niche right now.
This is not to say that in the future some webmasters may try to create a separate Young Adult niche in addition to the Teen niche.
Time will tell as it always does!
_