Help With Vet Visit

I'm very new to this and not sure if this is the right place to post, but I'm not sure where to go about this.  I have severe OCD and my cat Pitufo is a big part of my life.  He's 13 years old, but despite his age he's just as playful as he's ever been.  He still runs around and his eating/drinking routine is normal.  However, I have noticed that he has a black lump on his chin.  I researched it and didn't find anything serious so I've let it be, but now his lip is starting to droop.  It's on the same side as the spot and I'm going to bring him to the vet as soon as possible.  I am just very concerned about what they may say.  I'm aware that he may have some arthritis in his back since he's not fond of anyone petting it while he's standing on all fours (doesn't have a problem if he's sitting or sleeping), and due to miscommunication when he was being declawed he doesn't have any claws on all four paws so he has sores from fleas (which I treat as soon as I find them), but I'm worried that the verdict may be to put him down.  I'm not sure if it's my OCD blowing things out of proportion, but I'm very concerned this may be the answer.  He's still very happy and he doesn't show signs that anything is bothering him but I just can't get the thought out of my head.  How do vets handle elderly cats? Do I really need to worry?

From what you have said here I really don't think a vet will advocate putting your kitty down.  It is routine to do blood work on kitties regularly starting around age 10,  The blood work is just to check all the organ function but that doesn't mean that something bad will be found.  There are some things that in kitties, just as in humans, become more common as they get older.  Many of those things can be treated. 

The blood work also establishes a base line to compare future blood work results to.

Thank you for the response.  I'm feeling much better about taking him, especially with knowing now that this is all normal.  He's my first cat so I'm rather new to all this.. but thank you again!