Using the definite article the only works if you're talking about a specific group of young people.
Most of the young students at this school learn English.
If you're talking about young people in general, then you need the null article.
Most young people are impatient.
With an singular noun, you need to use “most of the …”, e.g. “he ate most of the cake”. Grammatically, the situation looks different. But there is actually a reason behind this. In “most of the young students at this school”, most qualifies a specific domain (the young students at this school). In “most of the cake”, most qualifies a specific domain (this one cake). In “most young people”, most qualifies the non-specific group of all young people. Since there is no specific domain, there is no definite article.