I'm going to oppose this amendment.
I'd like to respond to Mr. Davies' comments. How can a person reasonably believe that another person is of a given age if he took no reasonable steps to find out? I mean, what is reasonable in the circumstances will depend on the circumstances.
First of all, selling a joint at a party is already illegal. Second, you have to be responsible for what you do. If you're giving drugs to someone at a party, you should have a pretty good idea of how old they are. If you don't, then you shouldn't be doing it.