Perfect answer, Cherie, once again.
I bought one a couple of years ago who was only 6, but had also never been ridden in an arena, but you could put a monkey on his back and he did trails beautifully. Here was the issue I ran into retraining him. Because he was a "beginner horse" on the trails, he had never, ever, really been challenged or asked to do something on his own. Start slow. I think Cheries advise is perfect. And, BTW, I have a great arena AND trail horse now.