Learn Italian: Mio figlio ha tre anni. - My son is three years old.

In Italian, you express age using the verb avere (to have). Literally, this sentence translates to 'My son has three years.' Trying to use the verb essere (to be) for age, just because we do it in English, is a very common beginner mistake to watch out for!

Why is it just mio figlio and not il mio figlio?

Usually, you need a definite article (like il or la) before a possessive word, such as il mio cane (my dog). However, singular family members are a special exception. Words like figlio (son), madre (mother), or fratello (brother) drop the article when used with a possessive word like mio.

Is there a word for 'old' missing at the end of the sentence?

No, Italian doesn't use a word for 'old' when stating age. Anni is simply the plural of anno (year). Since the phrasing is literally about how many years a person 'has', stopping at tre anni (three years) is the complete and correct way to say it.