Depending upon her size and how well she eats you probably can give Ft.Worth a shot because she will be right at 8 months old, and I think if you start feeding her early along with some other additives along the way you will be ok come Ft. Worth. But on the bottom side of that don't get her to fat to soon.
My youngest son took a May 2, 07 Brangus heifer to Fort Worth last year and won his class. Just realize that in division you will be the youngest and not have a good chance of placing in front of the older calves. He had a blast and was so proud because it was only her second show. Go for it and remember it is their second year when they start to really shine.