The assumption that your teen years are the best, more shining and radiant part of your life—and that as soon as you reach legal majority you become a decaying thing that can only crawl in the rotting morass of adulthood all the way to your death—is very much a common Japanese idea... but it isn't at all a Western idea; more like the opposite, in fact. I'm surprised nobody felt offended by this strip and reacted with anger.
I'm also going to chime in on the side of people who are like...No, man. This isn't a Japanese idea, this is part of the human condition. It's universal across all cultures. There is a reason why, in the 80s, there was a huge surge of media about the 60s, and in the 00s, the 80s were big, and in the 20s, now the 00s are remembered fondly. In another 10 years they'll be waxing nostalgic about the 10s.
People making media about their youth to appeal to other people who want to remember THEIR youth is a huge part of culture.
I guess we've all already been mentally colonized by the Japanese. :P Come to think of it, I haven't read any rants against lolicon manga in a long time...
Why would we feel offended?
They call "decaying" people of thirty something. It's a pretty strong word.
It's clear that the adults are regarding themselves as decaying, and I'd be willing to bet the artist is being self-deprecating here, as well.
Almost nobody over 30 is going to be offended by a person referring to themselves as "decaying" because we all feel it everyday.