Not long ago, the IPO was regarded as a rite of passage into the adulthood that is the public stock market - a less sheltered and often more unforgiving environment, yet the all but inevitable goal.

Then a generation of tech startups privately valued above $1 billion rebelled against that adulthood, finding a steady source of private investment that allowed them to remain in a comfortable state of arrested adolescence. But that may be changing. Venture investors want the kind of exit an IPO brings. And more companies like Dropbox are starting to venture forth...