Is 17 to old for a thoroughbred?

If you and she are a compatible, safe pair and you can learn from her then she's a great match. 17 is not too old for your first horse. With good care, she has several more years of activity ahead and can likely help you learn a lot. You have only been riding 8 months and even if you are a fast learner, that's still something to consider that you need a reliable mount for your first horse. Just have a plan for her retirement in case something happens. All in all it sounds like a good match and she can jump and hack as long as she is comfortable and sound :-) I have a 32 y/0 mare who is great on the trails, but some hip issues have kept her from jumping the last 5 years... not too bad at all, and I really want her to be happy anyway.