Don't know PA. In CA, a person does not have to have any license of for that matter any training. You cannot run your own business. But a licensed plumbing contractor can hire anyone he wants. As a result, there are a lot of hacks working for the 1-800....plumbing companies.
Service business is steady if you have a good rep. New construction housing, and to some extent the remodel business.....is in the dumper.
In some areas there is good business to be had fixing foreclosure houses where the owners took...literally....the kitchen sink, bathtub, and some of the piping with them!
Getting started, you are competing with the illegal aliens, who will tackle any job someone is willing to offer them. Vent?.....you don need no steenking vent!