You are the only person introducting the concept of malicious intent to "spring" CD'ing on someone.
How far ahead do you need to plan to know that you have been cross-dressing for the last 10 or 15 years? Form the introductions I have seen that is a low estimate for someone in their mid 20s.
The vast majority of introductions here say something like "I have been cross-dressing since I was 6" although that age figure can vary by a coule of years either way. Are you honestly trying to tell me that someone who has been cross-dressing since that age and who is now in their 20s does not understand what cross-dressing is? I would buy the argument that many do not understand why they cross-dress, but I do not buy the argument that after 125 years you are unaware of what you are doing, or how you think about clothes.
I disagree with your reason why there are few 20 somehtings in this part of the forum, but I'm pretty sure that no-one in the Young Members' Forum is much over 25. My guess about why they are not posting so much in the main forum is that they get bored with the older members' threads. Either that, or maybe some are using Facebook to socialise with other CDs/TS's and don't feel the need of a specialised forum. In my experience, the younger cross-dressers and transsexuals have enough information to know that they are not freaks and also to know that they are unlikely to stop being CD or TS.
I don't know where you get this idea that you need to postpone your wedding for decades in order to be honest with your chosen spouse about an activity that you have already been doing for over a decade.