Whether other employees have non-competes is of no consequence – there is no rule that employers have to treat people the same. As to whether the non-compete is enforceable, that is too complicated to discuss here. Consult an employment attorney for further advice.
I can't speak to Pennsylvania law, because I'm not licensed there, but I can discuss general principles. Non-competition agreements are, under general principles of law, enforceable if they impose reasonable geographic and time limitations. There are a few places where they've been held unenforceable, but I have no idea whether Pennsylvania is one of those places.
Neither do I know why your continued employment with the company after signing the non-compete wouldn't be sufficient consideration to support the non-compete. I mean, if you don't sign it, they fire you. If you sign it, they continue to employ you. Sounds like consideration to me.
If your contract does not specify that you must work for them for a certain period, I don't know why you wouldn't be free to leave.
If I were in your circumstance and I had a job offer from another company, I would consult in person with a lawyer licensed in Pennsylvania to get an opinion about the enforceability of the non-compete in the specific context of the job I was expecting to take.
Don't take what I say here as legal advice as I'm not licensed in Pennsylvania and don't practice law there. What I wrote above is just my two cents on the facts you describe in light of general principles of law. If you need legal advice, please consult a lawyer who holds Pennsylvania licensure.
Mr. Thomas is spot on. You should have retained an attorney to review your non-compete before signing it. That being said non-competes are not favored by the courts but they are enforceable. Not all non-competes are enforceable however so you should not consult with and retain an attorney.
Nothing prevents you from leaving your current job. The non-compete limits what you can do after leaving.