I am 26 and already feeling that. It think it's not about being thirty, it's a condition that comes with becoming an adult and becoming immersed in work and/or family. I wonder who I'll talk to when I have children of my own. Maybe strangers on the internet... today I feel closer to those I talk to online than to those that I meet through my work life and social circle... I guess it has to do with becoming more demanding of others too. I had a lot of friends I can't stand anymore because their conversation bores me to death.
Your social circle will become the parents of your children's friends and classmates. You will have less in common with, and spend much less time with your friends who have not started families yet. This is a pretty common trend among young adults.I wonder who I'll talk to when I have children of my own