Mijn opa is zeventig jaar.

Dutch often leaves out oud when someone's age is clear from context.

All of these are possible:

• Mijn opa is zeventig.
• Mijn opa is zeventig jaar.
• Mijn opa is zeventig jaar oud.

They all mean the same basic thing. The version with oud is the most explicit; the shorter versions are very common in everyday Dutch.

After an exact number, Dutch normally uses jaar:

• één jaar
• twee jaar
• zeventig jaar

So zeventig jaar is correct.

Jaren is used more for an indefinite or general plural sense, for example:

• al jaren = for years
• in die jaren = in those years

So in an age expression, jaar is the normal form.

Yes, it is correct and natural.

That said, these are all common:

• Mijn opa is zeventig.
• Mijn opa is zeventig jaar.
• Mijn opa is zeventig jaar oud.

The shortest version, Mijn opa is zeventig, is especially common in speech. Adding jaar or jaar oud just makes the age expression a bit fuller.

Yes. Opa is the normal everyday word, like English grandpa.

Dutch also has grootvader, which means grandfather, but it sounds more formal and is less common in ordinary family speech.

So:

• opa = everyday, warm, natural
• grootvader = more formal

Because mijn already fills that slot.

In Dutch, a possessive word like mijn works like my in English:

• mijn opa = my grandpa

You do not say:

• de mijn opa

Just as in English you say my grandpa, not the my grandpa.

No. Mijn stays the same with masculine, feminine, neuter, and plural nouns.

Examples:

• mijn vader = my father
• mijn moeder = my mother
• mijn kind = my child
• mijn grootouders = my grandparents

In informal speech, mijn is often reduced to m'n, but the full form is what learners should know first.

This is the basic Dutch statement pattern:

• Mijn opa = subject
• is = verb
• zeventig jaar = complement (the age)

So it follows a simple subject + verb + rest order.

If you turn it into a yes/no question, Dutch usually inverts the subject and verb:

• Is mijn opa zeventig jaar?

You use niet:

• Mijn opa is niet zeventig jaar.
• Very commonly: Mijn opa is niet zeventig.

Use niet, not geen.
That is because you are negating the statement about his age, not a noun.

A rough English-friendly guide is:

mijn opa is zeventig jaar
≈ mine OH-pah is ZAY-vun-tikh yaar

A few useful notes:

• mijn: the ij sounds somewhat like the vowel in English eye
• opa: stress on the second syllable: o-PA
• zeventig: stress on the first syllable
• the final g in zeventig is often a guttural Dutch sound, not an English g

Pronunciation varies by region, but this will help you recognize and say the sentence.