We’ve been talking about tenure a lot lately, the subject of years repeatedly bubbling to the surface as people have aged and come and gone and everything else that comes with the ticking of the clock.

Last night, a birthday party. Two, in fact: One I attended, the other, across the city, I couldn’t, both significant milestones. Today, a friend left to begin life in New York, and Tuesday, an acquaintance does the same.

All of these occasions have been simultaneously celebratory and mournful, the joy of moving on and forward mixed with the sadness of an era’s passing. Even my friend’s son turning 21 had a tinge of the bittersweet, recognition that even the young get old.

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I’ll admit to a tinge of jealousy at all these new phases the people around me are experiencing. My life has been stable perhaps to a fault as of late, and the milestones I can see on the horizon are all ones that are markers of reaching a stage closer to the end of life than the beginning, at least when viewed through the prism of youth.

A friend of a friend came to my place for the first time the other day, and expressed admiration for its aesthetic. When he inquired how long I’ve lived here, he was shocked by the answer—he had likely just started high school when I bought my home.

It wasn’t all that long ago that I was him. It wasn’t all that long ago that I was new in this home, new in this city, or new in the town before that. But while my mind is capable of processing things through the filter of geological time, all those things seem so long past.

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Listening to a podcast the other day, I learned that life expectancy data suggests I have another 46 years ahead of me, which puts me roughly 40% of the way through this life. The way the human brain experiences time, however, compresses each year more than the one before, stripping each of just a little more significance.

There are still new places to go. There are still new things to do. But I fear that the things that seemed within reach when I was younger keep slipping further into the future. Today included an argument about how soon self-driving cars will be ubiquitous, and even I, the optimist in the situation, put the answer a decade from now.

The last man left the Moon more than a decade before I was born. Sometimes, I wonder if the next will arrive before I die.