I see a lot of late 20's in your list, and a lot of companies that started with one person writing some code on a computer.
Like I said, even 28 is not a kid or young adult (eg 16-22). You're spitting distance from 30, and most of those companies I listed were founded by 30 somethings (~35 is not barely into adulthood). It's reaching. Founders like Jobs, Zuckerberg and Gates fall onto that very young list, few others do. Add up the average age of the people I've listed.
> and a lot of companies that started with one person writing some code on a computer.
That describes few of those companies in fact.
For every famous very young founder, there's a lot more counter examples:
Markus Persson (30, Mojang), Craig Newmark (43, Craigslist), James Goodnight (33, SAS), John Sall (28, SAS), Diane Greene (43, VMWare), Mendel Rosenblum (36, VMWare), Bill Coleman (48, BEA Systems), Evan Goldberg (35, NetSuite), David Sacks (36, Yammer), Jack Smith (28, Hotmail), Sabeer Bhatia (28, Hotmail), Chad Hurley (28, YouTube), Andy Rubin (37, Danger; 41, Android), Rodney Brooks (36, iRobot), Jeff Hawkins (35, Palm), Niklas Zennström (37, Skype), Janus Friis (27, Skype), David Bohnett (38, Geocities), Bill Gross (40, GoTo.com/Overture), Subrah Iyar (38, WebEx), Min Zhu (47, WebEx), Wilfred Corrigan (43, LSI), Joe Parkinson (33, Micron), Aart J. de Geus (32, Synopsys), John Moores (36, BMC Software), Vivek Ranadivé (40, Tibco)
26 year old dudes are adults, full stop. And in my experience, they don't think nor act like teenagers whether they settled down or not, whether they married or not, whether they kept old hobbies or not.
People "reach adulthood" (to use your words) when they take on real responsibility. Some people never do this. Some do it at an early age.
Some things that often cause people to grow up fast include having children or serving in the military.
(Those are opinions. Source: Me.)