People who are currently 25 have less babies than people who were 25 ten years ago. People who are currently 30 have less babies than people who were 30 ten years ago.[0]
People living familyless lives longer means that the cars will come later in their lives. Having children and not have a car is a very tough proposition, considering the volume and mass of babystuff you have to handle.
Of course the cars might not come at all if people continue to chose solitude over offspring.
[0] http://www.childtrends.org/?indicators=fertility-and-birth-r...
Both of you seem to be describing the same thing in related, correct ways.
http://www.npr.org/sections/health-shots/2016/01/14/46281645...
I don't claim this is the case per se, nor that it explains the effects entirely. Just that it's a consideration and it might be unfair to say the thinking is necessarily "in much the wrong way".
Here's the real trend:
People who are currently 25 own less cars than people who were 25 ten years ago.
People who are currently 30 own less cars than people who were 30 ten years ago.
People who are currently 35 own less cars than people who were 35 ten years ago. (Note, these are the people who at 25 owned more cars than current 25 year olds)
Yes, when the people who are currently 25 turn 35 (aka the millennials age), they will own more cars than they did at 25. But all trends support the idea that they will own less cars than the people who are currently 35.
While not exactly purchases, I like this graph showing every age cohort is less likely to take out a car loan today than in 2003.
http://public.tableau.com/profile/joe.cortright#!/vizhome/Ca...
And also this graph about how many people have driver's licenses, every age cohort has gone down since 2008. And for young cohorts, it has gone down tremendously since 1983.
http://public.tableau.com/profile/joe.cortright#!/vizhome/Dr...