Mój brat ma trzydzieści lat.

Mój means my.

It changes form to agree with the noun it describes. Since brat is a masculine singular noun in the nominative case, the correct form is mój.

Compare:

• mój brat = my brother
• moja siostra = my sister
• moje dziecko = my child

So the form of my depends on the gender, number, and case of the noun.

Brat is the subject of the sentence, so it appears in the nominative case, which is the dictionary form.

In this sentence:

• mój brat = the person being talked about
• ma = the verb
• trzydzieści lat = the age expression

Because brother is doing the action of having, Polish uses the nominative form brat.

Ma is the 3rd person singular present tense form of the verb mieć, which means to have.

So:

• mam = I have
• masz = you have
• ma = he/she/it has

Here, ma matches mój brat, because my brother is he.

Polish expresses age with the verb mieć (to have), not with być (to be).

So Polish says the equivalent of:

• My brother has thirty years

not:

• My brother is thirty years old

This is completely normal in Polish. It works like this:

• Mam 20 lat = I am 20 years old
• Ona ma 5 lat = She is 5 years old
• Mój brat ma trzydzieści lat = My brother is 30 years old

So even though it sounds unusual from an English point of view, it is the standard Polish pattern.

Polish often omits subject pronouns when they are not needed.

The verb form ma already tells you it is 3rd person singular, and the noun mój brat clearly shows who the subject is, so on would be unnecessary.

You could say:

• On ma trzydzieści lat

but it would usually be used only for emphasis, contrast, or clarity.

Trzydzieści is the Polish word for thirty.

It is a single word, just as English has thirty as a single word. In this sentence, it is the number that tells us the age.

A few related numbers:

• dwadzieścia = 20
• trzydzieści = 30
• czterdzieści = 40
• pięćdziesiąt = 50

So trzydzieści lat means thirty years in the age expression.

Polish uses different forms of year after different numbers.

With age, the pattern is:

• 1 rok
• 2, 3, 4 lata
• 5 and higher usually lat

So:

• ma 1 rok
• ma 2 lata
• ma 3 lata
• ma 4 lata
• ma 5 lat
• ma 30 lat

That is why trzydzieści lat is correct.

A very useful rule:

• numbers ending in 2, 3, 4 usually take lata
• but numbers ending in 12, 13, 14 take lat
• numbers ending in 0, 1, 5, 6, 7, 8, 9 usually take lat

Examples:

• 22 lata
• 24 lata
• 25 lat
• 31 lat
• 32 lata
• 34 lata

Lat is the genitive plural form of rok in this kind of expression.

After many numerals in Polish, especially 5 and above, the noun often appears in the genitive plural. That is why you get:

• pięć lat
• dziesięć lat
• trzydzieści lat

So even though learners often memorize this as a fixed age pattern, there is a grammar reason behind it: numerals like trzydzieści require this form.

Yes, Polish word order is more flexible than English word order.

The most neutral version is:

• Mój brat ma trzydzieści lat.

But other orders are possible for emphasis, for example:

• Trzydzieści lat ma mój brat.
• Mój brat trzydzieści lat ma.
  This one is much less neutral and sounds marked or literary.

So for normal everyday speech, Mój brat ma trzydzieści lat is the best choice.

A rough pronunciation guide is:

• mój ≈ moo-ee / mooy
• brat ≈ braht
• ma ≈ mah
• trzydzieści ≈ very roughly tshih-JESH-chee
• lat ≈ laht

A few useful notes:

• ó in mój sounds like u.
• rz in trzydzieści is pronounced like a Polish zh/sh-like sound depending on context; for learners it is often easiest at first to imitate the whole word rather than build it sound by sound.
• ści at the end of trzydzieści sounds like a soft shch/sshch plus ee to English ears.

Stress in Polish is usually on the second-to-last syllable, so:

• trzy-DZIEŚ-ci

Yes, it is completely natural and standard.

It sounds like normal everyday Polish. A native speaker would immediately understand it, and it is exactly the kind of sentence used to talk about someone’s age.

So as a model sentence, it is very good:

• mój brat = subject phrase
• ma = correct verb form
• trzydzieści lat = standard age expression