DS16 has a gf 18 who he's been seeing for 18m or so. He is pretty mature for his age - youngest of 4 - and she isn't, and they look similar, so it doesn’t seem like a maturity or power imbalance.
He is only just 16. His siblings all had the rule of no staying overnight with someone until they were 18.
They both went to Reading festival in the summer, each camping with their friends, and I had the conversation with him before he went about mates before dates etc as I didn't want him spending the weekend with his gf instead of friends when he had arranged to go with them. Recently I was told via his friend's mum that he'd stayed in his gf tent.
Gf lives some distance away so we do say she can stay over any time. The other night she was sleeping downstairs and DS in his room upstairs; I woke up at 6am hearing someone opening/closing a door and on stairs.
I spoke to DS today. I said he is not permitted to sleep overnight with his GF until he is 18 (this was the rule we had for our other DCs) and I asked him about Reading and the other night at home. He said he'd stayed in GF tent one night at the festival and that the other night at home he fell asleep with her downstairs.
I've said he's broken our trust and she can't stay over and he can't have so much freedom now, because he's broken the rules and lied by omission.
Have I been totally naive here to trust him? Is it fair to apply our family rule of age 18 before sharing a bed with someone overnight ? AIBU to give DS less freedom now?