Well, they don't ask you for a birth certificate or anything.
We made our each of our daughters wait until they were actually 9 before we let them do the horse ride. But, that was mainly because I am cheap, and was trying to put it off as long as possible
Let me warn you, though, that if your daughter is really into horses, even at 8 1/2, she will find this experience to be very lame. You will be taken out by a guide with a group of well trained, calm, bored (I would say near comatose) horses, who have each done this a million times. Your horse will walk slowwwwwly behind the horse in front of you along a designated path. I don't even think you need to hold on, as your horse will barely be moving for most of the ride.
Only one of my daughters ever took riding lessons, so the trail ride was a nice experience for the girls who had never ridden a real horse before.
When you get to the Fort, you might want to stop by the trail ride area and watch the folks going out and coming back. Your DD may decide that she would rather not experience this, and I'm sure that you could find a better place to spend your $$ at the Fort (like renting a golf cart or the HDDR).
TCD