If you're going to buy alcohol and you're under 40 take your ID... PERIOD!!!
I work at a retail store. In the last 24 hours I have had or witnessed another coworker who had to deal with a person attempting to buy alcohol. NONE of which had their ID or the person with them of drinking age had their ID.
In the first instance the woman didn't have her ID with her. She was around 26 or so. And then tried to get some guys, I think, who were with her to buy it. Nope, can't allow that either. Then she came back in (with her ID) and I waited on her. She said the employee had refused her the alcohol because she had her two year old with her. I'm thinking, 'What?' (I thought that sounded awful strange and didn't really believe her. I learned the whole story from the coworker after she left.)
Second incident a 40 (possibly 50) something mother comes in to buy alcohol and has her 21 year old daughter with her. Daughter doesn't have her ID with her. Mom said, "it's mine, I'm not sharing." I explained to her something like I'm sure that's true but your daughter is of drinking age and is with you so I need her ID too. Here's why (I didn't actually explain this to her but will the next time.)... If I were to sell to you and then a cop outside saw you both walk out together with the alcohol and asked for her ID to confirm her age and she didn't have it not only would your daughter be in trouble but I would too and I could lose my job and possibly face criminal charges.
The third instance she was close enough to 40 that I went ahead and allowed her purchase but reminded her that our store policy is if you are between the ages of 21 and appearing to be 40 we need to see your ID.
And if someone with you is under 21 and not your child I can refuse the sale. Just because you say it is your alcohol doesn't mean you won't buy it and share later. If something happened and you got caught and that purchase was traced back to our store and to me I would get in trouble and so would you.
And no I also can't sell the alcohol to someone else with you either. If one person doesn't have their ID I cannot sell to any of you.
I once carded a group of about eight women even though only three of them were buying alcohol because they were all together which I could easily tell. And I could not make a single sale until I verified everyone was of legal age. They were a very friendly group of ladies who made my job very easy and all complied.
EDIT: I did have an instance where I had a mother with a 17 year old son. When I asked for his ID (he looked older) she explained he was 17 and her son. I allowed the purchase and later asked management about it. They said it was at the cashier's descretion.