Questions & Answers about Wie alt bist du? Ich bin zehn.
How do I pronounce the number zehn?
In German, the letter z always makes a "ts" sound, like at the end of the English word "cats". The h in the middle is silent, but it acts as a signal to make the e sound longer. So, zehn is pronounced a bit like "tsayn".
Do I need to say "years old" after zehn?
You don't have to! Just like in English, you can drop the extra words and simply say Ich bin zehn (I am ten). If you want to say the full phrase, it would be Ich bin zehn Jahre alt (I am ten years old), but the short version used in this sentence is very natural and correct.
Why do we use bist in the question and bin in the answer?
Both of these are forms of the German irregular verb sein (to be). The verb changes to match the person doing the action. You use bist with du (you), so du bist means "you are". You use bin with ich (I), so ich bin means "I am".
Can I use Wie alt bist du? to ask anyone their age?
You should only use this exact sentence with friends, family, or children. It uses the informal word for "you" (du). If you ever needed to ask a stranger or a formal acquaintance their age, you would use the formal "you" (Sie) and ask Wie alt sind Sie?