It depends on the horse. I've seen horses that look old at 12, and horses that look young at 20. Age is just a number 😉 Have a chat to your instructor/mentor about it, you need to consider her fitness, health and of course your own plans for her
At least work her once a week, but if she is overweight (and you are trying to loose weight) Then I suggest a couple times a week.
It really depends on how fit she is and if she is sound. I would consult a vet and perhaps an instructor. Each horse is unique so there isn't a rule that applies to what you can with all of them. But there is a 17 year-old gelding where I ride that is stil worked every day and has more energy than most horses! So don't let her age scare you, it depends on other things as well.