Je mu dvacet let.

Because Czech normally expresses age with a special pattern:

být (to be) + person in the dative + number + form of rok (year)

So Je mu dvacet let is the standard Czech way to say He is twenty years old.

Czech usually does not add a separate adjective meaning old in this basic age statement. A form like dvacetiletý exists, but that is more like twenty-year-old as a description, not the most basic way to state someone’s age.

mu means to him. It is the short dative form of the pronoun on (he).

So in this sentence:

• je = is
• mu = to him
• dvacet let = twenty years

A native English speaker often expects something like he, but Czech uses to him in this construction instead.

The longer, stressed form is jemu.

In this age pattern, Czech normally does not use on as the subject the way English uses he.

Instead, the person whose age is being given appears in the dative:

• mu = to him
• jí = to her
• mi = to me

So mu already tells you who the sentence is about.

If you do use the full pronoun, it is usually for emphasis or contrast:

Jemu je dvacet let, ale jí je osmnáct.
He is twenty, but she is eighteen.

Normally, no. mu is a short unstressed pronoun, and Czech short pronouns usually go in second position in the clause.

So the neutral word order is:

Je mu dvacet let.

Here, je takes the first position, and mu comes right after it.

If you want the pronoun at the beginning for emphasis, use the full stressed form:

Jemu je dvacet let.

That sounds much better than Mu je dvacet let.

The basic noun is rok = year, but its form changes after numbers.

• 1 rok
• 2, 3, 4 roky
• 5 and more let

So:

• jeden rok
• dva roky
• tři roky
• dvacet let

That is why dvacet let is correct.

This also affects the verb in age expressions:

• Jsou mu tři roky.
• Je mu dvacet let.

Because Czech does not copy the English pattern here.

With higher numbers such as dvacet, Czech uses the singular verb in this kind of expression:

Je mu dvacet let.

But with 2, 3, 4, you usually get the plural:

• Jsou mu dva roky.
• Jsou mu tři roky.
• Jsou mu čtyři roky.

So the verb form depends on the Czech number pattern, not on the English idea of plural years.

Not in standard Czech when you are simply stating someone’s age.

The normal pattern is:

• Je mu dvacet let.
• Je jí dvacet let.
• Je mi dvacet let.

So it is best to learn age with je ... let, not with mít (to have).

Yes, very often in everyday speech.

You will hear:

Je mu dvacet.

That is completely natural when the meaning is obvious from context.

Still, Je mu dvacet let is the full form, and it is the safest version for learners to use.

You keep the same pattern and change the dative pronoun or noun.

Examples:

• Je mi dvacet let. = I am twenty.
• Je ti dvacet let. = You are twenty.
• Je mu dvacet let. = He is twenty.
• Je jí dvacet let. = She is twenty.
• Je nám dvacet let. = We are twenty.
• Je vám dvacet let. = You are twenty.
• Je jim dvacet let. = They are twenty.

With names:

• Tomášovi je dvacet let.
• Marii je dvacet let.

So the age phrase stays the same; you mainly change who the age belongs to.

Very simply:

• Statement: Je mu dvacet let.
• Question: Je mu dvacet let?
• Negative: Není mu dvacet let.

A very useful related question is:

Kolik je mu let?
How old is he?