if you're 4 and he's half your age then he is 2 which means he will always be 2 years younger than you, so if you're 100 then your brother would be 98!
If you are 4 years old, and your brother is halve your age then your brother is 2 years old. This is a two year difference between you two. When you are a hundred, then your brother would be 98 years old then.
