I'm still under 18 and always had a 100% success rate going to lounges on my own. List:

GC North and GC South (3 times on the same day :/) -> April 2017
CCR; DUB Lounge -> December 2016
AA also let me in their secondary JFK Admiral's Club earlier this year.

At the lounge, I've seen that after the boarding pass is scanned: there is a tick to indicate lounge access eligibility. Considering that I wasn't 18 and I saw the tick come up on their side, lounge access (for T5 at least) should be fine regardless.