why do some men not age fast when they are in their 40's while others look really old ?

Some people live their lives harder than other people.  I go to my high school reunions every year, and we were 58 at our last one.  The partiers - smokers, drinkers, etc., had aged a lot by then, but many of the "goody two shoes" had not.

It seems what is inside of you gets written on your face as you age.  A woman who had been attractive as a girl, but with a sour personality looked sour at 58.  Others still looked good - not good for their age, but good, period.  It takes all kinds!

Because they don't eat healthy or exercise enough. 

I am 32, and I look about 21-22. I have excellent genetics, and I exercise regularly, don't drink or smoke, limit carbs, eat organic and a vegan diet. 

The rate at which people age visibly is a combination of genetics and lifestyle, and to some degree certain grooming practices. 

some older men look very good for their age.  They never stopped working out and eating right.  No drugs.   No stress.  No bs.

Some of it's lifestyle, as others have said. Alcohol, other drugs and tobacco age you, and so does high stress, bad diet and a sedentary lifestyle. But a lot of it's genetic, too.

how you spend your 20s and 30s matters...

Multiple factors including stress. 

it depends on the guy