Roughly 1995 by my watch. And it's sickening.
The good news is that I have some young family members who are raising toddlers now; they are super "hipster" and live in hip neighborhoods in Portland. They go to Disneyland once a year
(ostensibly to come visit Uncle TP2000, but I know it's mainly cause I live 7 miles east of Disneyland and I have a pool), and they
NEVER bring or rent strollers. The infants are held in backpack things, the toddlers and kindergartners walk all over the parks. The toddlers are occasionally carried for short periods of time by their very fit and fun young parents. When I bring up the subject of strollers and how I marvel that they are raising their children without them, they flatly respond that strollers are for people who don't know how to raise their children correctly and they would be mortified to have their hipster friends see them in public pushing their kids in a stroller.
So, at least there's hope, blossoming forth from the Great Pacific Northwest, just like Starbucks and Nordstrom did. Seasons One thru Three of Portlandia are available on Netflix now. And don't miss Season Four of Portlandia on the IFC network, premiering January, 2014 for more instruction on how to be hipster!