Learn German: Mein Bruder ist jetzt achtzehn.

It is exactly like English: acht (eight) plus zehn (ten). Unlike sechzehn (sixteen) and siebzehn (seventeen), which drop letters from their base numbers, achtzehn keeps the full word acht. Notice it only has one 't' in the middle!

Do Germans use the verb 'to be' for ages, just like in English?

Yes! To talk about age, German uses the verb sein (to be). So you say Mein Bruder ist achtzehn (My brother is eighteen). If you have studied French or Spanish, you might remember they use 'to have' for ages, but German keeps it simple for English speakers.

Why is jetzt in the middle of the sentence instead of at the end?

In English, we often put time words like 'now' at the very end of the sentence. In German, time words like jetzt usually come right after the verb. That is why it is ist jetzt achtzehn (literally 'is now eighteen').

Why is it mein Bruder and not meine Bruder?

Bruder (brother) is a masculine noun. Since it is the subject of the sentence, we use the basic masculine form for 'my', which is mein. You would use meine if you were talking about a feminine noun, like meine Schwester (my sister).