My niece has been drinking quantities since she was 12 (it's down to her mother who wanted a drinking buddy) who easily downs 3+ bottles of wine every night but is functioning.
When she is with her dad he has never been drunk and has had a hot toddy for flu about 5 times max in his life because he needed to keep working as it was his busiest time, but to stop her sneaking it behind his back or getting dodgy stuff, he has made it that she can drink at his house as long as she funds it herself and he will buy it for her. He goes off with the shopping list and often "forgets" stuff on it and spends the money on nibbles to go with it.
I've bought her a bottle of vodka which I brought to her older sisters wedding as she was determined she was going to drink anyway, she had brought some herself but I disposed of it after a taste as it was like paint stripper, I would far rather her drink proper stuff if she is going to rather than knock off stomach stripper.
I decanted it into 4 x 250ml bottles, I gave her one. I know if she had got the litre bottle she would have downed it, it was a litre bottle of paint stripper like I took off her.
She has a high level of tolerance and is now 16, she can have fun without it but likes the way it makes her feel and her mum offers her a glass of wine a couple of times a night and anything stronger if she has it. She is constantly being exposed to it and it pushed upon her as she is called names by her mum if she won't take any.
I never was exposed to alcohol at home and when I was 17 and moved out of home within 2 months, one night myself and a friend were admitted to hospital - concussion made worse by alcohol poisoning - we were feeling very rough and every member of staff we came into contact with reading us the riot act.
I now consume less than 10 units a year, it's a real treat and friend while she drinks moderately now. It taught us both it wasnt good and did horrible things to us.
If you want to, go ahead a glass or 2 with dinner but despite all above no I wouldn't recommend giving a 15-year-old a full bottle. Fun and good times can be had without alcohol, they are not linked.