Do they?
I have seen beards on men of all ages, from the peach-fuzz teener who’s dying to look suave to the hoary old coot who looks downright Biblical. And clean-shaven gents of all ages, as well. Except in certain cultures where men’s facial hair is more than a matter of personal choice, I don’t see a basis for this generalization. You might as well ask why men over 55 wear golf shirts.