In many ways the age difference is less than 12 years - Justin's very intelligent and emotionally pretty well adjusted (as much as any 17yr old, anyway) whereas Brian is still struggling with forming adult relationships.
The age of consent in the UK is 16 - the "Justin" character in the UK series was illegal and I did find that uncomfortable.
Age is such an arbitrary way of measuring competence, given that individuals vary so very much, but I suppose we have to draw a legal line somewhere. Once that age is reached, presuming the individual is mentally competent, and providing there isn't actual coercion, I'm not sure that the opinion of other people should matter.