Reading the article, the feeling of needing milestones doesn't resonate with me. But the feeling of "what next" after buying a house and getting a good job do, which makes sense as I've been working towards them for years. So do the higher level goals of travel, giving back, making new friends, learning new things etc.
That being said, the "what next" just comes later for people who have children (hence empty nest syndrome). And the goals she listed are pretty universal: lots of my friends who are mums still want to travel, make friends, learn new things, give back, celebrate (which is described as inviting friends round for a bbq or going out for breakfast).
It's nice to read an article by a child free person on being child free. But I wonder if the feelings are just general human desires.