@jtd871;
-Just after the last time your parents lifted you up for the last time
-The moment you realised the world wasn't all fun and games
-Around when you left your childhood home for your own
-And other depressing moments in retrospect.
In reality, nobody should ever truly "grow up". They should just grow. We change due to the experiences we encounter, the effects people have on us, the health, social, spiritual and emotional pressures we go through. Everyone deals with them differently, some choose to bottle it up, some choose to vent, some choose to run.
I chose to never truly "grow up". I enjoy playing, be it with computers, cars or even toys like Lego and RC stuff. We live in a world now that is accepting of these things - we can be in our mid 30's, 40's or what have you and still enjoy many of the things we did as children or adolescents, without it being socially unacceptable as it was in days of old (see Comic Con, YouTube, etc etc). This is a privilege that previous generations never had - there was always the pressure to "be an adult" and throw away fun in pursuit of arbitrary goals of 2.5 kids, a family home in the 'burbs, a 9-5.
Grow how you want, not how everyone around you thinks you should, i.e. "grow up". Life is a gradual journey, not a step-wise mission to hit certain milestones by a certain age.