Asking for help.
As for the attributive clause here, “stage” means “a period in one’s life” , but why we need where-clause here? Can we use when-clause?

Their child is at the stage ___ she can say individual words but not full sentences.

Question

Asking for help.
As for the attributive clause here,  “stage” means “a period in one’s life” , but why we need where-clause here? Can we use when-clause?

Their child is at the stage ___ she can say individual words but not full sentences.

Answer

Yes, "where" and "when" both work.  I prefer "where" because I think of a stage more as a place than as is a time.

For example, suppose Billy is studying mathematics.  You could ask his teacher, "where is Billy in his math studies?".  "When" would not make sense because the question is about position, not time.  The question is the same as "at what stage is Billy in his math studies?"