it definitely can be done at 18, i know a few people who have started at the college less than 6 months after their 18th birthday. some people do get all the way through first time, but by the same token a lot of people get through second or third time around and it has no effect on their chances of making it through to being a valid controller.
i know some people say that the first time they applied it was all a bit of a step into the unknown and as such felt they didnt do as well as they could, and as such were more ready for it the next time around.
my advice would be to take on board what people say and opinions, but then remember that its YOU against the standard, not anybody else. if you have the ability on the day, you will get through, regardless if everyone else there gets 100% or 1%.