My dad has been a norwood 2.5 for as long as I've known him. He's now 64 and still has the same norwood 2.5 he had when I was about 6 years old.
He was extremely lucky. Not everybody with male pattern baldness will end up as a nw7, but hair loss is progressive. Some men will go from nw1 to nw6 in a few years, for others the progression can take decades.