How old is jojo(the actor playing them) supposed to be in seusical ?

I am unfamiliar with that show except I guess it has something to do with Ted G.

I thought Jojo was one person, so your question about 'them' plural, indicates that Jojo was a number of people, which makes no sense. 

The character is listed in the cast as 'young boy'.  No specific age is mentioned.

The age of the actor is irrelevant.  Any actor can play that role if the casting director thinks they could convincingly play a 'young boy'.

your question is as nonsensical as Seuss' work itself.

do you want to know the age of Jojo, or the actor playing Jojo?