I also think that when I see people of that age married or with kids. But I think it’s just because of our different life experiences.
I opted to enroll in a PhD right after graduating and so, at 30, I still feel like my life isn’t at a point when I can start thinking about kids or marriage. But I know a lot of people enter relatively stable jobs as soon as they graduate university (or high school, although in my circles everyone went to university - it’s not as expensive as in the US here). I can understand people in that position starting to think about family earlier than me.