I think this is perfectly on point: "So, in essence, you are probably ok with the discrimination....but it has to be specifically against the person of whom the discrimination is against. You can turn down an under-18er, but not the over-18er they are with. You can kick out a smelly drunk, but not the non-smelly/drunk that he is with."
But let me try to rephrase. You are ok with the actual discrimination, i.e. discriminating based on age....it just needs to be directed at the correct person, i.e. the person underage. So you approve the actual discrimination, but might ban the association-discrimination.
Again, no problem discriminating against a smelly homeless guy for being smelly and/or homeless, but booting out the non-smelly/homeless guy who is with him should be stopped....because he isn't the one being discriminated against.