The clickbaity title put me off a bit. Does me not having children or a car mean I'm not grown?
You realize in the conclusion of your piece that it's not that Millennials can't grow up, but that what defines growing up has changed. We're letting go of past generations' rigid attachment to family and ownership milestones. Having those "unequal experiences" you refer to in the subtitle isn't necessarily a bad thing.
I'm in my 30s and was able to travel the world, build my career, develop as an individual, and care for my physical and mental health. Personally, I believe those are more telling signs of adulthood than having an American suburban lifestyle.
A better framing would be to say that Millennials are on average having more trouble accumulating capital and assets. But I guess that doesn't make for as catchy a title.