When I was 4 years old, my brother was half of my age, now I am 70 years old. How old is my brother?

He is 68.

Even though he is half your age, that doesn't mean that he will be half your age all your life. That just means he will be X years younger.

If you need an example:

Your brother was two when you are four. So when you become 6, your brother will be come 4. The fact that he is half your age doesn't really matter once you start growing up, because he will never be half you age again.