When there is consequently more to do, more to experience, more people to meet, more to discover, more novelty to novel at. In short, the modern era has more of what makes the childhood experience what it is, enough to spill over into what was previously considered adulthood.

I sometimes ponder what would happen if we did discover the secret to substantial life extension or even immortality; if this phenomenon of trying to stretch that delightful halcyon glow of childhood wonder - what I've heard the Finns call lapsuus - would extend into decades or centuries or perpetuity as a result. It would if nothing else provide a plausible explanation for the behavior of characters like Q from Star Trek.