For some reason, can't reply to your comment so I will do so here.
You're using rhetorical sleight of hand, shame on you.
If you could tell someone was a minor teenager or not just by looking at them, they wouldn't require ID for alcohol or cigarettes. Or, did that common sense escape your mind?
Top left looks like she's in her forties judging by her defined cheekbones, what appears to be crow's feet at the start of her eyes, and the sleight weight around her neck.
Top right looks like she's 17-21. Very bright smile, well defined features, very athletic, salon-styled hair. Typically, you don't see all of those features rolled into someone who is prepubescent.
Bottom left is the toughest. My vision isn't too good as I had eye surgery in August and am waiting until December for my other eye to be corrected, otherwise, features are fuzzy for the most part. THat being said, this photograph is taken at a zoomed in level moreso than the other three. I stand by my judgement that she looks early to mid-twenties in this image. But, this is only one image. Her cheekbones are largely pronounced and has well-defined laughlines which is something that only comes with age.
But I say again:
The reason why we check for ID when selling cigarettes or alcohol is because no one can tell whether someone is 17 or 18, 20 or 21.
Anyone with half a brain should know that if they resemble a teenager at all - ask questions.