Now, I have no issues with under-18s drinking, but I do have issues with under-18s being drunk (anyone being drunk tbh), and this would appear to be an "it's OK to be drunk" type of invite.

If there are 5 kids going then the "no sharing" thing may be OK, but to put it on an invite like that... hmm. I don't know, it just seems to be an alcohol party with no other activity. What happened to a weak punch and a sleepover, or a murder mystery game? Or a few songs on the CD player and people dancing and playing swingball in the garden? Can't you tell I went to wild birthday parties at 16 LOL.