Ему пятьдесят лет.

Because Russian expresses age differently from English.

Literally, Ему пятьдесят лет is closer to To him, there are fifty years than to He is fifty years old.

So Russian uses:

• ему = to him (dative case of он)
• not он = he

This dative structure is standard for saying someone’s age:

• Мне двадцать лет = I am 20.
• Ей тридцать лет = She is 30.
• Им сорок лет = They are 40.

In the present tense, Russian usually does not use a verb equivalent to is/am/are in simple statements like this.

So where English says:

• He is fifty years old

Russian simply says:

• Ему пятьдесят лет

The linking verb is understood.

But in the past or future, Russian does use a verb:

• Ему было пятьдесят лет = He was 50.
• Ему будет пятьдесят лет = He will be 50.

Лет is the genitive plural form of год (year), and it is the form used after many numbers.

Russian uses different forms of год depending on the number:

• 1 год
• 2, 3, 4 года
• 5–20 лет
• and then the pattern repeats:
  • 21 год
  • 22 года
  • 25 лет
  • 50 лет

So пятьдесят лет is correct because numbers ending in 5, 6, 7, 8, 9, 0 usually take лет.

Because in this sentence пятьдесят is just the numeral fifty in the basic form used for counting.

The important change here is in the noun год → лет, not in the numeral.

Russian numerals can decline in other contexts, but in a basic age statement like this, пятьдесят stays as пятьдесят:

• ему пятьдесят лет
• ей пятьдесят лет

It is completely natural and standard Russian.

Native speakers really do say age this way:

• Мне десять лет
• Ему пятьдесят лет
• Сколько тебе лет? = How old are you?

This is the normal Russian pattern for age, not an artificial classroom construction.

Sometimes, yes, in casual speech, if the meaning is obvious.

For example, Russians may say:

• Ему пятьдесят.

This still means He is fifty.

However, for learners, it is safest to use the full form:

• Ему пятьдесят лет.

That is the clearest and most standard version.

The usual question is:

• Сколько ему лет? = How old is he?

This uses the same structure:

• Сколько = how many
• ему = to him
• лет = years

So:

• Сколько ему лет?
• Ему пятьдесят лет.

Yes, Russian word order is fairly flexible, though some versions are more neutral than others.

The most neutral order is:

• Ему пятьдесят лет.

You may also hear:

• Пятьдесят лет ему.

But that sounds more marked, expressive, or context-dependent.

For learners, use:

• Ему пятьдесят лет.

You keep the same pattern and just change the pronoun:

• Мне пятьдесят лет = I am 50.
• Тебе пятьдесят лет = You are 50.
• Ей пятьдесят лет = She is 50.
• Нам пятьдесят лет = We are 50.
• Вам пятьдесят лет = You are 50. / You all are 50.
• Им пятьдесят лет = They are 50.

So the age expression itself stays the same; the pronoun changes.

A few points may help:

• Ему is pronounced roughly ye-MOO
• пятьдесят has stress on the last syllable: pyat-dee-SYAT
• лет is stressed and pronounced like lyet

A rough pronunciation of the whole sentence is:

• ye-MOO pyat-dee-SYAT LYET

Also, the initial е in ему sounds like ye at the beginning of a word.

Not in the normal way English does.

English says:

• He is fifty years old

Russian does not normally say the equivalent of He is fifty-year-old in ordinary age statements.

Instead, Russian simply uses the age construction:

• Ему пятьдесят лет.

There are adjectives related to age, but they are used in other contexts, not as the standard way to state someone’s age.