Would this work?

I think it might with a different dynamic to see how Young Leonard would have grown up. I am only a casual watcher of Big Bang Theiry and haven’t watched this season so I don’t know the character’s biography. Is Leonard an only child? I think he is. Who was his father? I take it his family was more wealthy than Sheldon’s family.

Who would play the younger mother?

A while back on Facebook I saw a gag ad for Young Dwight with a picture of a kid who would look like a young Dwight Schrute. Would love to see that.

But a Young Leonard Show, why wouldn’t this work?

Leonard does have siblings. A running gag on the show is that Leonard's psychiatrist mother Beverly is disappointed in him because he is not as accomplished as his sister and brother.
I don't think a Young Leonard could work because Beverly is sort of an older, female, less likeable version of Sheldon and she and Leonard's father were miserable together. Thereby making Leonard miserable, too. None of that sounds very appealing for a sitcom.

Well consider the following, what makes Sheldon so interesting and by extension Young Sheldon is his never-ending attempts to organize the universe according to his vision. Leonard and for that matter the rest of the Big Bang characters are attempting to understand the universe around them and then find their place in it. The character in young Leonard's universe is his mother, who is for all practical purposes a kind of female version of Sheldon, the show would have to be called Young Leonard's Mother.