Yeah, my mother always used to make a tick on the wall which shows how I much grew since I was like 5 years old.
It seems I've lost at least 1.5 cm of height (my maximum height was 174 cm).
I guy I work with has shrinking man's syndrome (not really sure it's nick-named this... his nick-name). He says it's a form of osteoprorosis. His father has lost 6 inches to 8 inches in height. I'm not sure how much my friend has lost, but it is a bit noticeable. He is now taking osteoprorosis meds.
He's 55, and his father is in his 80's. 20 sounds a little young for this... but who knows.