I think 13 should be the limit, as at that age, people start to be more independent, more in control of what they do, more sceptical of things, so in this case, I don't think a 13 year old would be totally convinced of all the praise he/she is receiving, to the point where, unlike tonight, that person wouldn't think they had the competition in the bag, only for all those hopes to come tumbling down.
Yes, children make mistakes, and children can learn, but doing so in front 10m people, in a very highly pressurised/contrived set-up, as well as being up against bonafide worldwide talking point (I think putting her amongst this lot was definitely intentional, to get rid of potential threats), isn't a good way of learning mistakes.