It all depends on the horse and how interested she is in learning a new job.
I think we hold the record for the oldest horse being trained to drive--25 years old! He was a retired eventing horse that was very sad that he never got to go out and work....but riding was too tough on his body. So he drove! And took to it like he'd been doing it all his life (of course, when he was 5 and saw carriage horses in the distance, he would panic and try to run....so obviously some maturity helped).
However, I have also heard of horses trained when they were older(past 6-8 or so) that were always unreliable because they never quite 'took' to driving and felt their ideas of running away were much more important than listening to the driver. So....it depends on the horse. But I would say, no she is not too old to at least try to train her to drive. If she wants to drive, she can definitely be trained to drive.