Question

Remove too
He is too young to join the army

Answer 1

he is young to join army

Answer 2

The right answer for the above question is as follows:

He is so young that he cannot join the army.  

Do you know why the prepositions are used here?

Because when you are going to avoid a word from the sentence, it needs to be replaced with other word given the same meaning.

Here in the given sentence, that is not possible so the entire sentence got altered to make it correct.