Learn Italian: Il museo ha duecento anni. - The museum is two hundred years old.

In Italian, you talk about age using the verb avere (to have) instead of the verb to be. Literally, this sentence translates to saying that the museum has two hundred years.

Is duecento always written as a single word?

Yes. In Italian, numbers are combined into a single word without any spaces or hyphens, so two hundred is simply written as duecento.

Since anni is plural, why doesn't the ending of duecento change to match it?

The word cento (hundred) and its multiples are invariable in Italian. This means their endings never change to match the gender or number of the noun, so the word stays duecento instead of becoming duecenti.