I’m 16, fly in CE regularly alone and am Gold. They do still do the £30 voucher if you’re denied access or ask at a check in/customer service for one (one day I ended up with £90 in vouchers which they quickly put a stop to). If they’re busy in the GC they’re unlikely to notice but a get around is at the North lounge they have the self service machines so are even less likely to notice - I’ve only ever been denied entry in GC South and GF.

Certain managers/staff are quite firm while others could not care less less but all are right in saying technically she’s not permitted. There’s nothing immediate on the system that indicates under 18, as they always ask for my passport when they’re unsure but I’d recommend using a mobile boarding pass as I often get ‘YP-16’ printed on the paper ones.

Also worth noting the only location that has ever caused me any bother is LHR T5, anywhere else she should be fine as they won’t be BA staff (LGW and T3 have also always been ok). Remember if one lounge refuses entry to try the other one, I don’t think I’ve ever travelled when every T5 lounge has refused me and success was high before I started to be remembered.