I've also heard that it was 14, but it was on a programme about teenage mums so I can't quote it as gospel!
I waited til I was 19 and my BF was 18...thinking back his mother had no say in the matter, he just invited me round to stay so she cooked me tea and wished us goodnight...we didnt consult her at all, which was very rude. And then she extended the same 'courtesy' to his sister who was 5 years younger than him so 13 at the time...her boyfriend was 16. And she got pregnant at 14. The problem my MIL has(yup, I married that BF) is that she never knows how to treat her kids; one minute she treats them like adults, another moment like kids.
I think thats an important key: you have to decide based on your DD (or DS) whether you deem them mature or not in any given circumstance.
My sister met her now ex-BF at 16 and she asked my mum if she could share a room. I'm pretty sure they did allow it, although I was at uni at the time...sharing rooms with boys quite a bit, but not sleeping 'with' them, just next to them.
The first time my DH stayed over at my house, my mum had that day been admitted to hospital (happend several times a year) and I was very upset. Although we were very much in the infancy of our relationship my DH/BF chose to stay with me and look after me even when I needed to cry and just be upset. Neither of my parents was consulted on this decision as neither was there to ask.
I think thats what it boils down to : individual circumstances. If you know the boy well enough and have faith in your daughter then allow it if you feel comfortable. If you know the boy and have faith etc but DONT feel comfortable, then have a discussion with your daughter about this. Be honest and open to her requests.
That being said, I admire those on here with principles and while I may not entirely agree, I think its good to hear that your have them. I think its respectful to adhere to ones parents wishes even if you do not have the same ideals. Looking back, I would have asked my mum to allow my BF to stay the night eventually, and she most likely would have wanted more time to get to know him herself and for the relationship to progress more before agreeing.