Learn English – the difference between “one” and “a”

Without the article, "What is time?" is most likely asking for the definition or meaning of "time". If you asked someone this question, they would likely respond by explaining what the concept of "time" is.

Here's a simple definition:

Time:

1. [uncountable] the thing that is measured in minutes, hours, days, years etc using clocks

Or a link to the Wikipedia page:

Time is a measure in which events can be ordered from the past through the present into the future, and also the measure of durations of events and the intervals between them. Time is often referred to as the fourth dimension, along with the three spatial dimensions.

The second question is a general question asking for the current time of day. If you asked someone "What is the time?", they would look at a clock of some sort and respond with the current time:

A: What is the time?
B: It's 10:30 AM.

This uses the second definition of "time" (from the same source as above):

2. [singular] a particular point in time shown on a clock in hours and minutes:

This could also be rephrased as "What time is it?"

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.