Learn English – When to use most things and when to use most of (the) things

Most has two closely-related meanings.

1 the largest in number or amount
2 more than half / almost all of somebody/something

As that OALD link says, the most is often used for the first meaning - but informally, the is often omitted. It's never used with the second meaning.

Suppose The Queen of Hearts were to ask "Who ate my tarts?"...

A1: "The knave ate the most"
A2: "The knave ate most"

If several courtiers ate one tart each, but the knave ate two, then either answer could validly be given, since he ate more than anyone else (but note that A1 is a "very slightly formal" usage).

But if the knave actually ate more than half (but not all) of the tarts, only A2 is correct. Though the Queen might not understand that intended distinction, since the could be omitted anyway.

It's worth making the point that the is never included when most simply means very...

"I am most grateful for your help"

...but interestingly, whereas in...

"There were 50 people there, at [the] most"

... the is normally omitted, it's almost always included if we add very as an intensifier...

"There were 50 people there, at the very most"

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.