It depends on where you start - that's one important variable.

For example, I took up swimming at age 69. I made some progress, learned a proper stroke from a lesson, and kept on swimming regularly (for almost 3 years now). However, it is just these past few months, when i decided to push a bit harder and try for better endurance that I have seen some pretty good improvement - enough so that I am seriously considering a sprint triathlon next September. This was a case of starting at pretty much ground zero and improving from there. If I had been a swimmer all my life, I wouldn't be showing so much progress on a percentage basis, but, overall, I would likely be a MUCH better swimmer than I am now, even with my pushing myself.

Ditto to my progress in resistance exercises, although I have been doing them for years. I can still improve - as in doing 72 pushups in a row a couple of weeks ago (a new PR for me), and 30 x 3 body dips this morning - also a new PR at almost age 72.