That's a really complex question and I think it's rather hard to comprehend electing to have an amputation unless it is required to save a life.
I wonder though if you're seriously considering having your leg amputated - wouldn't a knee replacement be a better alternative? I thought the reason they don't do them at a young age is that they aren't likely to hold up the remainder of your life. Doesn't seem like that would matter if it's that alternative or have your leg amputated.