I give it an 8.
I can't say with certainty this is first, but this episode is among my earliest recollections of seeing Star Trek
on television. And over the years I've heard a lot of people dump on this episode, but whenever I watch I find myself liking it on some levels.
Firstly there's early TOS' execution which is often quite good. The performances are also usually there. And perhaps on some level I can identify just enough with Charlie because I was about eleven years old when I first saw this. Even after all these years I find this is one story with a youth in it that doesn't make me cringe and gag.
That said if there's anything that really dates this story it's seeing the Enterprise
crew struggle with trying to explain the facts of life to young Charlie---it's very much of its time since this was a subject matter that was dicey to dance around on television of that era.
It would be easy to play it safe and just give this a fair rating, but in all honesty I find myself liking it a little more than that.