I too have thought about this from time to time...
I guess the best explanation I have is that it would take way more time for an adult to do a whole-life-memory-search for relevant information, compared to a young child... In needing to search for much more information in about the same amount of time, things move quicker.
As these searches are a big part of thinking about anything, perception of time changes too...
I guess, reflecting on this, you sort of need to work faster to keep up. Your success in reaching your desired perception state depends upon that. Your happiness on that success...
If I had any advice, I guess it would be, don't try to slow time down for too long. Try and keep up, but don't worry too much if you don't reach your true destination every time. As long as you maintain your sense of care about your life, you're still on the right track. And if you ever get off track, you can always get back on track... You've been there before, and know where that track is...
Hope that helps... If not, perhaps a more specific instruction of what you need help with would be helpful for future advice...