A baker is someone who bakes breads or cakes.
"Making lots of dough" is an expression that means making lots of money. It could be understood literally, so a person "making lots of dough" could be referring to a person making lots of bakery dough, something used to make cakes. This joke is playing on the term "making lots of dough". 
OLD BAKERS never die , they just stop making lots of dough .