Sorry for the late reply but only just seen thread lol

No way is 9/10 too old to compete! I have just bought a 16.3tb Mare, 12yo, and I am starting to compete her next year! She also sat in a field for 2 years, couldn't tell you why because I was never told, and other than the occasional hack she lived out 24/7 eating! She was also in a bad way when I got her i.e. underweight and had no muscle at all (she wears a 5'9 rug!!!)

I looked at her once, sat on her for couple minutes and then bought her because I knew I had to take her home and had a gut feeling it would be a good decision. Turns out it was!!
Yes, she needed physio and few other bits so did take a bit of time but in 5 months I have now got her up to weight (if not a little fat), muscle buidling up, coat in good condition, riding her 6 days a week, some of which out on hacks (which she is brilliant on may I ad) and she is also now jumping over 3ft while being let loose in the school and has an unbelieveable amount of talent!
I dont think you can think something through too much, try and find out as much infor as pos but also go with what your gut is telling you.

Let us know how u get on