Ich kenne sein Alter.

In German, kennen is used for being familiar with a person, place, or thing, or for knowing something as a fact in the sense of being acquainted with it.

So Ich kenne sein Alter means I know his age in the sense that this information is known to me.

wissen is more often used with:

• clauses: Ich weiß, dass er 30 ist.
• question words: Ich weiß, wie alt er ist.
• specific facts stated directly

In many situations, both ideas are close, but in this sentence German naturally uses kennen with the noun phrase sein Alter.

Sein Alter is in the accusative case because it is the direct object of kennen.

The verb kennen normally takes an accusative object:

• Ich kenne ihn.
• Ich kenne die Stadt.
• Ich kenne sein Alter.

Here:

• Ich = subject, nominative
• kenne = verb
• sein Alter = direct object, accusative

Because sein here is a possessive determiner meaning his.

It works like:

• mein Buch = my book
• ihr Name = her name
• sein Alter = his age

Since Alter is a neuter noun and is in the accusative, the form stays sein Alter.

For a neuter noun:

• nominative: sein Alter
• accusative: sein Alter

So this form is exactly what we expect.

Because all nouns are capitalized in German.

So:

• ich is not capitalized in the middle of a sentence
• Alter is capitalized because it is a noun

This is a basic but very important rule in German spelling.

In this sentence, Alter means age.

German das Alter can mean:

• age in general: sein Alter
• old age in some contexts

Here, because of sein Alter, the meaning is clearly his age, not old age.

German main clauses usually follow the verb-second rule.

In Ich kenne sein Alter:

• Ich is in the first position
• kenne is in the second position
• sein Alter comes after that

This is the normal word order for a simple statement in German.

The dictionary form is kennen.

Kenne is the first-person singular present tense form:

• ich kenne = I know
• du kennst = you know
• er/sie/es kennt = he/she/it knows

So the sentence starts with Ich kenne because the subject is I.

That would usually sound unnatural in standard German.

German prefers:

• Ich kenne sein Alter.
• Ich weiß, wie alt er ist.
• Ich weiß, dass er 30 ist.

So if the object is the noun phrase sein Alter, kennen is the natural choice.

If you want to use wissen, German often expresses the idea differently, usually with a clause.

Usually sein means his, but it can also mean its, depending on what it refers to.

Examples:

• sein Vater = his father
• sein Ende = its end

In Ich kenne sein Alter, the usual meaning is his age, because we normally understand sein as referring to a male person already mentioned or understood from context.

You would say:

Ich kenne ihr Alter.

Here:

• sein Alter = his age
• ihr Alter = her age

Be careful: ihr can also mean their in other contexts, so context matters.

A simple pronunciation guide is:

ikh KEN-uh zine AL-ter

A few notes:

• ich has the soft German ch sound, not an English k
• kenne has a short e
• sein sounds like English zine or zayn
• Alter has stress on the first syllable: AL-ter

A more IPA-like version is:

[ɪç ˈkɛnə zaɪn ˈʔaltɐ]

Yes, usually it implies that you know how old he is, or at least that you know the information about his age.

In normal use, Ich kenne sein Alter suggests:

• you know his age
• his age is not unknown to you

If you want to be more explicit, you could say:

• Ich weiß, wie alt er ist. = I know how old he is.
• Ich weiß, dass er 40 ist. = I know that he is 40.