我二十岁。

Just drop your age after 我 (wǒ). In this case, it is 二十 (èrshí) for twenty. The word 岁 (suì) acts as the measure word for years of age. You can use this in almost any sentence where age is relevant. It is like a verbal ID card. Just don't forget the 岁 at the end or you're just saying 'I am twenty' (which could mean twenty apples!).

Use it when meeting new classmates or colleagues. It is perfect for filling out forms or talking to a doctor. If you are at a bar and the bouncer looks skeptical, this is your line. It is also great for making small talk with elders who love to ask about your life. It is a safe, standard, and clear expression.

Avoid using this if you are trying to be mysterious about your age. In some professional settings, you might not volunteer this info unless asked. Don't use it if you are talking about the age of an object. For a 20-year-old car, you would use different words. Also, if you are talking to a toddler, they might use simpler 'finger counting' logic instead!

In China, age is often a point of connection. It helps people determine where you fit in the social hierarchy. Knowing someone is twenty helps others know you are likely a university student. Age is also tied to the Chinese Zodiac. People might ask your age just to figure out if you were born in the Year of the Dragon or the Rat. It is less of a secret than in some Western cultures.

If you want to sound more casual, you can just say 二十了 (èrshí le). The 了 implies you have already reached that age. If you are almost twenty, say 快二十了 (kuài èrshí le). To ask someone else, say 你多大？ (nǐ duō dà?). For a more formal vibe, like in a job interview, use 我的年龄是二十岁 (wǒ de niánlíng shì èrshí suì).