Learn Portuguese (Portugal): Tu tens onze anos.

In English, we say we are a certain age, but in Portuguese, you have years. To talk about age, always use the verb ter (to have). So, Tu tens onze anos literally means 'You have eleven years'.

I'm learning the verb ter. Why is it tens and not 'tes'?

The verb ter is irregular. A regular '-er' verb for tu would just add '-s' to the stem, but ter changes its spelling slightly in the present tense. Tens is the specific irregular form you must memorize for tu (the informal 'you').

Is it mandatory to include the word tu here?

No, it is perfectly fine to just say Tens onze anos. Because tens is only ever used with tu, the listener already knows exactly who you mean. Including tu just adds a little bit of emphasis.

Would I use this exact sentence to speak to an adult stranger?

No, tu tens is informal and mostly used with children, friends, and family. If you were speaking formally to an adult, you would use the 'você' form: Tem onze anos.