I have noticed a difference starting aroung age 30. People start taking much more serious care of themselves - stop the all night parties, develop life goals, drop the addictions, etc. Few people under the age of 30 start their own companies.
Note the prevalence of startups and new tech ventures with the under 30 set and the popularity of YCombinator.
And most of this activity is pure drivel. This is the group that is also most susceptible to peer pressure (infatuated with blogging and even more with social media) and believes that social activities are a value creation activity in themselves.
The clients I have had in this age group have all been fools. Their judgement has been absolutely terrible, every single one.
I think you pointed out a while back on several occasions that the brain is under development until the mid 20s - most of us are biological adolescents well into our 20s.
