Need help with a 15yo

I'm about to have my 18th birthday (18th of may) and we are going to Barcelona the 12th of June, all of the participants are in the range of 16-17yo except one, this dude will turn 16 the next month (July), but when we'll be there he will still be 15, now, I don't want to keep him out of this but still, since it's a DOUBLE 18th birthday (me and a friend are born the same day), I can't like, restrict everyone because of one of my friends, so the question is, will it have a huge difference from 15 to 16yo? will we have problems when we want to enter in like some club?