Hmmm....just because I had seen so many posts here about the tiny trundle, I stretched out on it on my last solo visit, just to see how small it really was. I was not the least bit uncomfortable and I am 5'7". Unless the 12 year old is really tall, I would think she would be fine. My son slept on it when he was about 10. Is it as comfortable as a real bed? No, but it beats sleeping on the floor!