Mi hermano mayor tiene veintiún años.

In Spanish, age is expressed with tener (to have), not ser (to be).

So:

• Tiene veintiún años = literally He has 21 years
• Natural English translation = He is 21 years old

This is one of the most important differences between English and Spanish.
You would say:

• Tengo 30 años = I am 30
• ¿Cuántos años tienes? = How old are you?

Using es here would be incorrect.

Because veintiuno changes to veintiún before a masculine noun.

• un año
• veintiún años
• treinta y un años

This is a special form of uno before masculine nouns:

• uno → used when the number stands alone
• un / -ún → used before a masculine noun

Compare:

• Tengo veintiuno. = I’m twenty-one. / I have twenty-one. (number alone, depending on context)
• Tengo veintiún años. = before the masculine noun años

The accent mark shows the correct stress.

Veintiún is stressed on the last syllable:

• vein-ti-ÚN

Without the accent, the pronunciation/stress would not follow standard spelling rules. This written accent is required in the shortened form before a masculine noun.

Compare:

• veintiuno
• veintiún años

The accent appears because the word changes form when uno is shortened before a noun.

Normally, age in Spanish is expressed with tener + number + años.

So the full structure is:

• tener + age + años

Examples:

• Tengo diez años
• Ella tiene cuarenta años
• Mi hermano mayor tiene veintiún años

In some contexts, people may omit años if the meaning is already obvious, but for learners, the standard and safest form is to include it.

In this sentence, hermano mayor means older brother, not necessarily big brother in size.

Mayor here is used to talk about age:

• mi hermano mayor = my older brother
• mi hermana mayor = my older sister

If you want to talk about physical size, Spanish usually prefers words like:

• grande = big/large

So:

• hermano mayor = older brother
• hermano grande could suggest a big brother physically, but it is not the normal way to say older brother

In Spanish, adjectives often come after the noun.

So:

• hermano mayor
• literally: brother older

This is the normal word order here.
Although Spanish adjective placement can sometimes change meaning or style, with mayor meaning older/elder, the usual position is after the noun:

• mi hermana mayor
• su hijo mayor

Because Spanish uses possessive adjectives like mi, tu, su, nuestro directly before the noun, without an article.

So:

• mi hermano = my brother
• tu casa = your house
• su coche = his/her/your car

English also does this: we say my brother, not the my brother.

Spanish does have longer possessive forms like mío, but those are used differently:

• mi hermano = my brother
• el hermano mío = the brother of mine / my brother (less basic, more marked)

For a simple sentence like this, mi hermano is the normal form.

Spanish does not use an article with a possessive adjective in this structure.

You say:

• mi hermano not
• el mi hermano

Also, age expressions do not use an article before the number:

• tiene veintiún años not
• tiene los veintiún años

So the natural pattern is:

• Mi hermano mayor tiene veintiún años.

Normally, no. To mean older brother, Spanish uses simply hermano mayor.

Mayor already contains the idea of older/elder, so adding más is usually unnecessary or nonstandard in this kind of basic description.

Use:

• mi hermano mayor = my older brother
• mi hermano menor = my younger brother

In Spanish, every noun has grammatical gender. Año is a masculine noun:

• el año
• los años

That is why the number changes to the masculine form:

• veintiún años

If the noun were feminine, the form would be different:

• veintiuna chicas = twenty-one girls

So the masculine noun años causes veintiuno to become veintiún.

You would change the family word, but the age part stays the same because años is still masculine.

So:

• Mi hermana mayor tiene veintiún años.

Notice:

• hermano → hermana
• mayor stays mayor
• veintiún años stays the same because it agrees with años, not with the person

Even if you are talking about a woman, you still say:

• tiene veintiún años

No. Veintiún is used before any singular masculine noun.

Examples:

• veintiún años
• veintiún libros
• veintiún días
• veintiún chicos

The same idea appears with other numbers ending in uno:

• treinta y un libros
• cuarenta y un días

So this is not something special only for age; it is a general grammar pattern.

A simple pronunciation guide is:

• veintiún ≈ bein-tee-OON
• años ≈ AH-nyos

A few useful details:

• The stress is on the last syllable of veintiún
• The ñ in años sounds like ny in canyon
• In much of Spain, v and b sound very similar

So the whole phrase sounds roughly like:

• bein-tee-OON AH-nyos

Yes. That would still be completely correct.

• Mi hermano tiene veintiún años = My brother is 21 years old
• Mi hermano mayor tiene veintiún años = My older brother is 21 years old

Adding mayor gives extra information about which brother you mean. If you have more than one brother, it can help identify him.