Juggling might be in the same vein as bicycling, or swimming. Learn it and it’s really hard to unlearn it. Or maybe like tying your necktie or shoe laces. You learn it once with more focus and then periodically if recalled you retain it.

Anecdotal, but I’ve learned how to flip the balisong over a couple days in my late teens at the cost of lots of cuts on my hand and fingers (more dramatic than it sounds really) without a guide. I haven’t had one in three decades, but I got my hands on one a year or two back and I was able to recall the motion and technique in only a couple tries without any cuts. Even today when I think about it, I can do the flick motion and my hand and wrist instinctively yearns for the weight of the cool steel.

I’ve watched the Man From Earth a couple times. Can only recommend.

However it doesn’t fit your description. Oldman says that his memory is basically limited. Just like any mortal’s. Only the brightest, most impactful memories are retained and the rest is a blur. If you are forty plus, you barely have memories of your childhood today, unless you have recorded them as soon as you could and rehashed them frequently. Same for him. As such, he is constantly evolving with the world mentally (and physically apparently).