Is your cat female or male? Just wondered why you did not refer to its gender?
What are you feeding your cat? You said your cat was jumping around on everything before about six weeks ago.
When you had your cat declawed, was he/she scratching all over the place?
If your cat is really lethargic, he/she could have the beginnings of a chronic illness that cats can get as they start aging. Perhaps chronic renal failure, diabetes, hypothyroidism or possibly even a tumor somewhere. Even just a cyst that is not cancerous can cause changes in a cats routine.
If this were my cat, I would definitely try to make an appointment at the vet's for a thorough examination, and blood work, and maybe even x-rays, if there is anything suspicious, such as a lump or swelling that wasn't there before. Maybe, since you have declawed your cat, it could be sad or bored without the front claws, and now there is not as much reason to run around and claw at wood or furniture, etc. But, since it has been six months since you said you declawed your cat, that doesn't seem likely that this would be the main reason for its lethargy since then.
Some illnesses can catch up with a cat that was normally active and then the cat may not feel well, and just lie around most of the time.
It could really be any number of things, or your cat may just be bored and unless you initiate playtime with it, he/she may just be showing beginning signs of age, and with no other cat playmate, it may just be feeling blue. I do know that cats have very many different feelings, and my own have gone through periods such as you describe at different times.
Best thing, though, is to get a good checkup as I said, and rule out any diseases first. If everything is negative, and your cat is given a clean bill of health, then you might want to spend more time trying to "play" or interact with your kitty.
Eight is really not that old, and you are right to be concerned.
God bless and good luck! Let us know if you have any further questions or concerns.