What are the consequences for an 18 yar old taking a 15 year old to a party?

If the party has alcohol then the conseunces can be very bad. Depending on your track record if you've never done anything bad you may just be let off with a warning or you could go to jail however if you do get caught by the local authorities just tell them you don't know the 15 year old and that he or she snuck into the party. That way the 15 will most likely just get a warning whenever they do this.