Being white could mean she's older than 17 as greys get lighter as they age. Be sure to have someone who is knowledgeable (vet) age her. She could be 25.
While this is true, being pure white could also mean that she's younger than 17.
The gray in this picture was not even 8 years old. He was born black but was nearly white by the time he was 3, pure "white" by 7.
How fast they gray, or how slowly, depends purely on genetics.
Anyway, melanomas are something to watch out for with grays.
However, if she has a few melanomas but is perfect otherwise, I would be pretty unconcerned about it.
My gray mare, who's now 28, is basically riddled with them. She has them in her mouth, under her tail, all over her udder, under her jaw, etc, and she does just fine. They'll probably be a problem in ten years, if they continue to grow (at this point they pop up, then stop growing), but that's a problem for when she's 38.
The ones inside her, that I can't see but know are there, are the ones I'm more concerned about.
But in any case, she's already "very" old and she made it this far with tons of melanomas! :)