Ekko seems to be like 15. Every other champion is either an adult (or young adult) or a child.
Is he the very first teenage champion in League of Legends history or am I forgetting someone?
http://img1.gm.gtsstatic.com/league-of-legends/ekko_5943_w620.jpg
Edit: 
are not teenagers, they're young adults (at least 20 y-o)
Ezreal and Lux are the only ones who could be teenagers,but like you said they are both most likely young adults. Well at least when it comes to human champs then he is the first teenager,I mean inhuman champs are kinda hard to judge(we don't know what a yordle teenager looks like).
was created to take the place of a teenage girl wasn't she?
seems like a teen
acts like a teen
If you don't consider Ezreal a teenager then you might as well forget about Ekko being a teenager.
As far as Lux goes, we don't know. "BUT SHE IS SUCESSFUL IN TEH ARMYRURURURUDUR" and Annie is a child that is used as a weapon.
There is such a thing as fiction.
willup is shown. the boy who actually controls the yeti is nunu and is confirmed a young boy/teen
right. 1st teenager.
Another thing you keep forgetting: Ekko's mental age and and corporal age might differ. His body seems to be rewinded in time, but his mind isn't (so he can remember the experience from another timeline and his enemy doesn't). In the video the difference is only a few seconds, but we have no idea how much he has used that power before and if it only was seconds...
somebody told me 
and 
were 16
