I suspect that bowling hasn't weakened the knee. After all, it's still exercise (albeit minimal). Everyday life without regular exercise will weaken the knee, and you are just beginning to notice it due to the bowling.
Most neoprene supports will keep it warm and offer some support. But, as someone who has had a partially torn ACL since 1993, I would suggest doing some physical therapy to strengthen the muscles around the joint.
The "heavier" the brace or support, the more limits to your range of motion. Some are actually designed to limit range of motion to decrease your chance of injury. Good luck.
Lane Carter, Strike Zone Pro Shops - Salt Lake City, Utah
Brunswick Pro Shop Staffwww.brunswickbowling.com