I would want to know why she's unbacked at that age, and if they genuinely have never tried why that is. Totally unbacked is bad enough, but a failed backing - as you seem to realise yourself - is far worse.
What is your feel about her from the ground? I grew up around the breed and owned one for years and more than many breeds I'd say you want to gel with one, particularly if it's a bright one - if you have a serious brain to go with the power and athleticism of a good ID then you need it working on your side!
Another factor to consider is is she registered? If backing her doesn't go well then a well bred RID mare would have breeding prospects.
Physically her age isn't a problem, it's the mental side of things that would concern me. That said if she's been driven she's used to taking instruction and working with people so it may be less of an issue. Even more than normal though I would say make sure each stage is very firmly established before going any further.