Oh there's always problems - it's just a question of when they pop where horses are involved
If the mare is truly calm and gentle and the training is clear, consistent and handled well, it should be a success. It just may take a bit longer to go through the training steps as she is older. As well, the training should be done by an experienced person as a grown horse may be more opinionated than a youngster. I assume she was used as a brood mare earlier in her life that is why she was not started before??