yirmilerinde ´in his twenties´

otuzlarında ´in his thirties´

kırklarında ´in his forties´

...

onlarında ´in his teens´ ?

I´ve never heard "onlarında" from anyone before. We use others but not this. We usually say he/she is around this age if a teenager.

No wonder you didn´t hear it. I invented it using analogy. Yani, there is no such word which you could use for every one from 11 to 19?

No, not really. We call teenagers genç kız/erkek(young girl/boy) or delikanlı (for boys).

Ok, it is a very artificial concept anyway, invented by those who want to market certain products and lifestyle for young people. Thank you, Faruk.

 We can say something like "Onlu yaşlarda" [ in age of 10´s]

Example ;

Daha onlu yaşlarda bir çocuk geldi işyerine bugün.

[ A child who is just in his /her 10´s came to the work [work place] today]

Oh, thanks for the addition, Kultigin. That´s a handy expression, too.