I was 5’5" forever. At a recent Dr appt, I asked to have my height checked. I’m now barely 5’4" - technically, only if you round up. I’m nearly 69, so I guess it’s to be expected at my age.
Who else?
Well, my waistline certainly isn’t. 
I’m 49 and still 5’10ish.
So what causes the shrinking? Spinal compression?
Yes. The cartilage between joints gets worn out and osteoporosis causes the spinal column to become shorter. I was nearly 6’ tall for a long time. Now I’m about 5’10".
I’m 55 and have lost a quarter inch since 48. My father has lost over four inches from 50 to 85, though some of that may be posture as he can’t stand as fully upright as he could.
I was rather more surprised to learn that I had grown almost an inch (20 mm) from age 19 to age 35. I had always thought that you stopped growing at age 18 or so.
Are you female? That sometimes happens with a first pregnancy.
I’m 45 (and male), and some time in the past decade or so I went from 5’ 11.5" to 5’ 10".
Yes. I was 5’9.5" in college. Fifty years later, I’m 5’8", maybe less.
I’ve been 5’10" since I was a teen (67 now) and at my last doctor’s appointment a few months ago, they measured me at 5’9". I mock cried and said, “I don’t want to be a little old man!”
No. According to my doctor at the time that’s quite normal for males as well.
I’m 64. That happened to me a couple years ago. I’m not sure I want to measure me any more. 
My husband got taller when he took up yoga.
Last doc visit still showed 5’ 10":, so no height change, but I’ve lost* 18 lbs. in the last 6 months. So I shrunk a bit I guess.
*Due to scolding by docs, etc. Decided to make some minor but consistent changes to satisfy them. Went from 198 to 180. Not great, but headed in the right direction.
Yup, I was 5’ 7" forever, then lost an inch.
I’ve herniated my L4 five times and have three bulging discs in my thoracic spine. 60 yo.
“Like a frightened turtle!”
(Had to be said. Will show myself out.)
I’m 62. Still 5-7.
Yep. I was 6’8" since I was in my early 20s. A few years ago every time I’d to the Dr., they’d tell me I’m 6’7".
I’m not even 50 yet, but I guess I had a lot of room to shrink.
5’7” all my adult life now the doc says 5’6.5”
Sshhh, no one else knows and I’m not telling.
Historically I’ve always been exactly 6 feet 0, but I topped out at 5 ft, 10 1/2 in when last I stood sock-footed on the scale-and-height device at the doctor’s office.
Will this force a change in your SDMB user name? 
I was 6’4" in my late 20’s. I can barely make 6’2" now. I’m 62yo
You can expect to be about 1 centimeter (which is close to .4 of an inch, since an inch is precisely 2.54 centimeters) shorter at the end of the day than you are when you wake up. If you want to know whether you are increasing, decreasing, or staying the same in height over the long term, always measure your height at the same amount of time after waking up. Some sources give the change during the day as a percentage rather than an amount. They say that you can expect to lose 1% from the beginning of the day to the end of the day.