I used to experience it pretty often - that feeling of being partly awake, but unable to move. I'd try to come up with some way to move an extremity - kick my foot or move a hand, but I had to work at it, and at times I would be able to 'break' the paralysis.
In recent years, I seem to experience it less often. The only factor, besides getting older, that has changed for me is that I moved to a new house. Since then, I experience it less. I also experience less of a thing I used to call "Flying", where I'd dream that I was flying around the house, and often doing so to check the front and back doors for intruders. The sensation of flying actually seemed enjoyable, and after a while, I sort of 'knew' that I was in that dream-state while it was happening. It was like 'hey, I'm flying in my sleep again - this is cool'.
I'm not sure whether the two were related, or were the result of taking sleep aids. But I still take sleep aids (usually Melatonin); even so I now seem to experience these sensations less frequently.