You know, Empress, I kind of got the feeling that we are trying to say the same thing, and all this is a misunderstanding coming from not being english native speakers.
If what you mean is that Federico Fernández Senderos sold Tintero to Charlotte because at 10 years old he already knew what its level was, and was alreagy old to have hopes of improvement, I agree with you.
Wealthy amateurs, like Charlotte, often buy horses around that age that the top riders have discarded because although they are very good, they are not the top horse they are wainting for. Of course at 10 years old you already know what to expect from a horse, and Tintero may not have the level to win the Grand Prix at a global champions tour (that's the level his former rider was competing in) but is still an amazing horse for an amateur.
Of course a 10 or 12 year old is a mature horse in the sense that you already know what to expect from him. It's not a project full of hopes like a young horse of 6 or 7. What I meant is that an 8 year old is young for top level competition in the sense that his best years are still to come, and he has't yet peaked as he will between say, 11 and 13. Most horses at 8 haven't started to show the best of themselves.
Of course top horses are so expensive that a lot riders try to sell them just before they start their downway hill, in order to get a good prize for them. A top rider won't buy his "next top horse" at 12, of course, that's old to buy a top horse. But it doesn't mean the horse is old for competition since his best years are yet to come.
And I was not trying to make the horse I ride pass for younger than he is, I swear, since my very own horse (I ride several others) is only 7!
Charlotte's purchase seems very reasonable to me. She's bought a horse who jumped the Grand Prix at top level international competitions. It must have been really expensive. But she's got the money! If she's a little bit talented a horse like this should allow her to have very good results at 1.30 amateur competitions.