A few days ago the NY Times had an article about how GM is trying to increase sales of cars to young adults. The Atlantic Cities picked up on it and expanded a bit about why auto ownership preferences have changed. I don't think any of the given explanations are wrong necessarily, but these stories miss what seems to me the most obvious explanations. For my money the most obvious explanation is that U.S. states have been making drivers licenses for minors much more restrictive since the 1990s. See this table for a complete list of restrictions. Back when I was young and foolish I got my license on my 16th birthday, and that was it. I was just as licensed as anybody else. Now, however, most states have some sort of graduated license, which means that if kids are following the law (and I suspect most do) then they get through most or all of high school without ever have full automotive freedom. Having a car you can drive around filled with friends at any hour is better than having a car you can drive alone during the day for most people.
Ultimately states have made it harder to get licensed, which should be reason #1 in the list of explanations as to why kids and young adults have fewer licenses. This isn't even an unintended consequence. It's what the policies were designed to do. It shouldn't be surprising that driving, like many other learned behaviors, is strongly influenced by the way one grew up. Preferences for walking and neighborhoods that do not require cars are likely derived from growing up without easy access to cars. This strikes me as the simplest explanation, and certainly a more direct connection than trading cars with smart phones (they aren't substitutes) or avoiding cars because they represent "adult" purchases (as the Atlantic article claims).