Нашому дідусеві сімдесят років, а дядькові — п’ятдесят.

Because Ukrainian usually expresses age with the dative case.

The pattern is:

кому? + скільки років
literally something like to whom + how many years

So:

• нашому дідусеві = to our grandfather
• дядькові = to the uncle

That is why you do not see the basic dictionary forms наш дідусь or дядько here.

That is just the normal Ukrainian structure for stating someone’s age.

Instead of copying the English pattern X is Y years old, Ukrainian normally says:

• Мені двадцять років.
• Йому сорок років.
• Нашому дідусеві сімдесят років.

A helpful way to think of it is:

• English: He is 70 years old
• Ukrainian: To him, 70 years [are]

It is not a word-for-word match with English, so learners usually need to memorize this as its own structure.

Років is the genitive plural of рік.

After numbers:

• 1 → usually рік
• 2, 3, 4 → usually роки
• 5 and higher → usually років

So:

• один рік
• два роки
• три роки
• чотири роки
• п’ять років
• сімдесят років

That is why сімдесят років is correct.

Because it is understood from the first part and does not need to be repeated.

Full version:

Нашому дідусеві сімдесят років, а дядькові п’ятдесят років.

But Ukrainian often omits repeated words when the meaning is obvious:

Нашому дідусеві сімдесят років, а дядькові — п’ятдесят.

This is very natural and common.

The dash marks an omission: something from the first clause is being left out in the second clause because it is already clear.

So the second part really means:

а дядькові — п’ятдесят (років)

The dash helps show the contrast and the ellipsis. It is similar to English writing something like:

Our grandfather is seventy, and our uncle — fifty.

In Ukrainian, this kind of dash is very common.

In a way, yes.

In the present tense, Ukrainian often omits the verb "to be". So there is no word corresponding directly to English is in this sentence.

That means:

• Нашому дідусеві сімдесят років
  literally follows a pattern like Our grandfather [is] seventy years old

You do not normally need to add є here. The sentence is already complete and natural without it.

No. Ukrainian often has more than one acceptable dative singular form for masculine nouns.

For example:

• дідусеві / дідусю
• дядькові / дядьку

So you may also hear:

• Нашому дідусю сімдесят років
• ...а дядьку — п’ятдесят

These are also correct. The forms in your sentence are perfectly standard and slightly fuller in style.

Нашому is the dative singular masculine/neuter form of наш.

It has to agree with дідусеві, which is also dative singular masculine.

Agreement here works like this:

• dictionary form: наш дідусь
• in this sentence: нашому дідусеві

So both words change because both belong to the same noun phrase.

Here are the basic forms:

• нашому → наш
• дідусеві → дідусь
• сімдесят → сімдесят
• років → рік
• дядькові → дядько
• п’ятдесят → п’ятдесят

This is useful because learners often need to recognize case forms and connect them back to the dictionary entry.

The apostrophe shows that the consonant before it is not softened and that the following я keeps its y-like sound.

So п’ятдесят is not pronounced as if п were soft. The apostrophe separates the sounds more clearly.

This is the same general idea you see in other Ukrainian words with an apostrophe, such as:

• ім’я
• п’ять
• об’єкт

For learners, the main practical point is: do not ignore the apostrophe.

Yes, Ukrainian word order is fairly flexible, but the version you have is natural and neutral.

Your sentence:

Нашому дідусеві сімдесят років, а дядькові — п’ятдесят.

puts the people first, then their ages. That is a very normal way to say it.

Other orders are possible for emphasis, but they may sound less neutral, for example:

• Сімдесят років нашому дідусеві...

That might be used in a more marked or expressive context. For a learner, the original order is the best model to follow.