There is no set answer - both are instigated by hormonal changes, and they vary from person to person.
A man can start to loose his hair in his tweties, or in extreme cases, his late teens, or he can start to thin and / or receeed in his fifties. Similarly, grey hair can start to show in the twenties, or not until the sixties.
Baldness is usually hereditary - but it does not always follow in familes - as in the Suchet brothers. Newsreader John has a full head of hair, his actor brother david is bald.
As someone who is folically challednegd, I must concur with beanebabe, there has never been a better time to be bald - get it cropped and forget about it.
The truth is, women are not so shallow as to judge a man by his hair, and a lot like a bald man.
Be what you are, and feel secretly smug at the 'Trumps' of this world.