This is a great thread, as I've often thought how I'm going to be able to keep myself skiing as long as possible. I started at 18, and stopped after having two children and money got tight. At about age 49 I started up again, and now at 57 I've definitely noticed changes in my physical abilities.
1. Keep yourself as healthy as possible. Exercise in the off season to try to maintain your strength and flexibility. You DO lose these things much quicker as you age, so don't take your physical strength for granted.
2. Avoid injury. If this means slowing down, not taking those last couple of runs, not going down a trail with sketchy conditions or giant moguls then so be it. We're going for longevity here!
You can push yourself to some extent, but don't overdo it. You want to be able to ski tomorrow (or next week, next month, next year) also. #1 is very important for avoiding injury. Recovery seems to take longer when you're older. Stop and rest when you need to, and stay hydrated.
3. Ski as many days as possible. Your skiing will improve, as will your conditioning.
4. Take care of your feet! Things that hurt can cause permanent problems. Don't try to make your boots last if they hurt because you think that you may only be skiing a season or two more. Visit more than one reputable boot fitter, as Gr8outdoors says, if you are looking for new boots.
5. If you are an intermediate, don't be afraid to take group lessons, or even private lessons, because you're older. You'll often find others in our age bracket taking lessons. The better you can ski, the more likely you'll be to avoid getting injured. AND the more fun you'll have!
6. Watch for osteoporosis. Catch bone loss early to start treatment early. This could be one reason why women don't tend to ski into older ages as frequently as men seem to.
7. Demo skis whenever you have the chance. You may accidentally find your ski soulmate, one that gives you joy and makes skiing sooo much easier. Our skiing style changes as our physical abilities change. I've met older women ski instructers who are high level skiers who've gone to softer boots and/or skis because they just feel better and make skiing easier.
8. Have fun! Plan new places to ski, buy that funky jacket, stand on top of the mountain and drink in the beauty of your surroundings. Go down that slope singing and do what makes you happy!