Teenagers only?

Had this shot of a 17 year old girl accepted - but a note that teenagers only (teenager) was not fully relevant. Can someone explain in what context I should use this keyword?
  

17 year olds should be teenagers.  I had something similar with a 19 year old, however in the cv an eighteen and nineteen year old isn't a teen (from now on to be known as twentyminustwos and twentyminusones respectively ).  David did give a good guide to which ages are which at the end of my thread.

Hmm! Going back 50 years to my Latin classes - wasn't 18 duo de viginti or something and 19 un de viginti? Along the lines you are suggesting Jo. Who'd have thunk it!