Young has only 1 vowels, so its comparative degree is direct+er; Less is generally used to modify the adjective/adverb prototype of more than two vowels, indicating that it is less than ..., such as its less important.

Two comparative words cannot be used together. If you want to express "he is much younger than me", use much to modify it, for example, he is much younger than me.

In addition, it is pointed out that the standard grammar should be that he is younger than me (am), because the comparison is between two people (him and me). However, both people say it's me in spoken English, and the convention is right.