I can't see any reason for any other age.  Actually to be honest I think both pot and alcohol should be legal at age 18 across the board.

Is pot 100% safe?  No, it causes health issues, as does alcohol, cigarettes, fatty foods and any number of other silly things many of us tend to do throughout our lives.  But at some point in time we need to recognize people as adults and let them make their own decisions, for better or for worse.

We have, collectively, decided that 18 is the cut-off age for "adult" in our society for nearly all legal definitions, so let's keep things consistent across the board.