I found that last year, the initial online application was probably the most competitive stage. Everybody I know who passed this initial application spent significant time on their application questions and a spent lot of time thinking about the best way to use their word count, and to stand out against the huge number of applicants.
Bearing in mind that the CTC staff have many hundreds of applications to read, and there was no prize for being the first application submitted, I'd take as much time as you need until you're happy with your answers!
4800 applications were actually submitted, (source: CTC assessment day), and a few hundred were invited to the assessments at Dibden, which ran from the end of November through into the beginning of December.
Successful applicants received emails inviting them to book an assessment day at The Base (Virgin's HQ, Crawley) from mid-late December all the way until the beginning of January. This meant, I believe, that some people had two or three weeks to prepare for the Virgin assessment, while others may have only had a week or so.
The Virgin assessments ran from the 13th-16th of January, if memory serves correctly. And yeah, the age range of successful applicants was (at the time) 20-30.
It seems however like the assessments might all take place slightly later this year, as people aren't hearing back from CTC until the 7/12/15?