I suggest getting a job. Behind a desk if possible. That way back pain and inflexibility are valid excuses to quit early; or better yet not even go out to the crag. Nothings screams "I'm weak" like a fat paycheck and a beer belly.
Tear knee cartilage when youre 15. Live past 35. Marry a beautiful climbing and out of doors person, then limit the time you spend out of doors with them. Sign a mortgage agreement. Work in an industry dependant upon an upswing economy. Wear a heavy toolbelt for 20 + yrs. Sustain enough nagging sport and work injuries to make getting out of bed with 6 hrs of sleep or less that much more special. Gain weight while consuming less healthy food. Spend too much time on the ground and the internet. ..
Taper, Baby, Taper....frequent rest days...lots of rest and relaxation....join a gym and go soak in the hot tub. Beyond that...get married, have kids, and get some debt...that should get you weaker than a new kitten.
For years I was looking to shed the climbing strength I had built up. It wasn't ginormous to start with, but I've had some serious success reaching my goal. For me, I went back to grad school, got married, had two kids and got a check list for things to do to the house. What did you do to get weaker and rid yourself of grip strength or just all around muscle tone? Looking for ideas because unfortunately I am re-motivated and starting to go to the gym.
May I recommend an overuse injury? Try this plan: Selectively remember how incredibly rad and awesome you were in your twenties, then set yourself up with a brutal training regime to get back to a level of fitness you might never have actually attained in your earlier life. You should hear some "pop" sounds on your first hard session at the gym. Top it off with a bitterly gobbled amuse-bouche of a half dozen cinnamon rolls at Lamar's.
This has been my plan for quite a few years. Works well!
The simplest way is to just grow older. Once past 75, the decrease in strength and ability occurs without any of the difficult measures suggested so far in this thread. Even so, I would be remiss not to mention that the time spent on elder care and grandchildren accelerates the process.
Spending two months in Baton Rouge settling a family estate while trying to run two businesses in CO. Too many flights, too much killer food, lousy weather all added up to a substantial loss of strength and increase in waist size. Took me the entire summer of 2012 to get it back.
My favorite would have to be falling in love, rejection, copious eating and drinking in a darkened room for lets say 6 months. That should result in 25 - 30 lbs which of course transfered mostly from muscle mass. It's not as fun as I make it sound but you will get weak!
If not that I'd agree with Rob.c just get old (now that's fun).