More to the point, why does it matter to us that the character is exactly seventeen years old? Unless it actually affects the story, there's no point in telling us. If you want a show, don't tell, for this type of detail, then it has to be organic to the scene. e.g.
The group all crowded into the bar. Except John. The bouncer stopped John. Can't fool a professional with a cheapie ID.
AND
He flicked through his Extension Math book – a few questions circled to give him the slight edge over his classmates.
Both scenes give you details about the scene, and roughly reveal the characters age.