I only once had a problem getting into a lounge when I was under 18, this was at LGW with an absolute dragon of a staff member who spoke to me in an awful manner- with one of those 'all teenagers are yobs' kind of attitudes.

However, this was only one occasion out of many, almost monthly lounge visits for me (between about 15-18) in a whole host of airports, mostly when I was Silver travelling in Y. In hindsight I'd now consider asking someone near the entrance who looks relatively approachable if no one is available on the FT meet up thread!