There are many milestones in life: leaving the parental home, getting into a serious relationship for the first time, getting a job, and so on. There is one no one tells you about, which sneaks up on you as you get into your thirties.
Suddenly, the phrase “when I was their age” enters your brain, and it refuses to leave. Reaching 30 means that there are adults in the world, who can drink and drive (though hopefully not in that order), who you have little in common with. “When I was their age,” you start thinking, “things were different.”