Congrats on the old timer jade
plant. I've had great success with mine for the most part (though it's only 6 or so years old...at least the cutting that I got, can't speak for the original
plant).
First off, if you're moving it from an under-lit area to a brighter spot, do so gradually or you will give your jade sunburn. Yes, I'm serious, and yes I learned this from experience. Jades are actually a lot more tolerant of getting less light than someone would think. Most of the year I keep mine outside, but it lives inside in the winter because of nights like this (26 degrees!). It's too big to keep on a window sill at this point, so I keep it on the floor in front of a window where it seems like it would get the most light. No complaints yet!
You pretty much don't need to water your jade in the winter if it's dormant as it should be. When your Jade is
growing it will grow a lot more if you water it more regularly (couple times a week?), but sandy soil is a MUST. The trick my friend taught me which enabled me to keep my jade alive since it was just a little thing is this: gently squeeze the leaves. If they feel soft or spongy, your jade needs some water. If the leaves are nice and firm, it's not thirsty. Of course it seems that if you over-water your jade the leaves will be soggy, but I'm not sure on that point as I've never done it (better to err on the side of under watering).
As for the spider mites, one thing that does the trick against most insects (maybe all?) as long as the infestation isn't too severe is to take a spray bottle, mix soap and water together and then give your jade a thorough spray down. Do that a couple of times a day and that should get rid of any nasty critters as the soap is toxic to them but not to your jade, and I believe the soap film is supposed to be harder to penetrate also.
You probably already know this, but jades are really easy to propagate, so if you're afraid your one and only 60 year old jade is going to up and die on you, take a couple of cuttings and stick them in some sand. If the original survives and thrives, pretty soon you won't even be able to tell where you took the cuttings from. Every year I lop off what seems like a good portion from my jade to start for friends, but everything grows in so fast to cover the gaps that it's always hard to tell where I've cut from.
