Finally, this is why when you watch people who start or try gymnastics moves as an adult, it often lacks a smooth quality of movement even when they complete the skill. I'm inflexible and out of shape too but my brain allows me to move more fluently upside down and backward because of intense early exposure during critical developmental periods. And probably better overall body development even in small muscles and tendons in the upper body particularly that rarely develop in the course of ordinary life.
When you're talking about people who are learning skills as adults, you're generally talking about folks who are training for very low hours most likely without proper coaching either . I'd chalk up a lack of smoothness to the lack of hours and proper coaching, rather than say it's just because they are adults. Put a kid in that situation and see how smooth their gymnastics is.