…when you hear that two friends need help moving, you can just reply “I’m too old for that shit” with a clear conscience.
But, all of those trips to the gym have kept me in pretty good shape and I still have a pickup truck and a trailer, so I still get the calls. I haven’t learned to say no, yet.
For me, the upside of middle age is that I no longer feel the need to explain myself to anyone. They can either accept me or kiss my ass.
The answer to the first half of your post is in the second half of your post. 
When I volunteered to help OpalCat move, my girlfriend reminded me of my age (46). However, considering all the times I’ve mentioned that I still fight in the SCA, it would look ridiculous to puss out now. (There’s a lot of stuff I don’t want to admit to being too old for.)
We’ve been known to offer to contribute to movers and/or pizza and beer to get out of the moving obligation now that we are middle age.
However, the other nice thing about middle age - your friends start figuring out the mover thing - there are only a few left who really expect (need) their friends to move them - and enough friends willing to throw in $50 to keep from throwing out their back that the “housewarming gift of movers” happens.
Still young enough to do it, but old enough to know how to do it right.
Heh. That moment came about for me when me and my friends all had full time jobs. I managed to scrape up money to pay movers, and so can they. The whole “give up your Saturday for pizza and beer” thing gets old after about age 22.
You realize that damn near nothing you do in life is going to have cosmic consequences.
I’m all in with the not givin’ a damn what others think anymore.
It’s also fun gettin’ to be “daddy” in certain situations.
Can we please get an age range here for those of us who are on the brink of what would be considered middle age. I’m in my mid-thirties.
Ew…
la la la la la … I can’t hear you… 
You’re still a whippersnapper. I’m talking 45-55 or so.
twicks, 52 … who will be 62 in 10 years … yikes …
Or…when you hear that two friends need help moving, you can just say, with a regretful half-smile, “Gee, I wish I could help, but my back…” and you reach around and gently grab your back in a Grandpa Lumbago gesture.
And get away with it. Whether you actually have back problems or not.
55 is only middle age if you expect to live to 110.
So we’re good, then.