When your bicycle(s) are older than the person waiting on you in the store.
Ain't that the truth!!! Hahahaha!
True story:
I went into my LBS a year or more ago with my mid-1980's Pinarello. I didn't want to leave it locked outside the shop. The young lad came up to me and asked if I was looking to upgrade. I told him no, but I did need a couple of tires. He showed me what he had and asked, "Can I show you a really nice bike?". Sure I said. He pulled a carbon bike down from the rack. Boy it sure was nice. I then asked him, "Will I be able to ride this bike 35 years from now?"
Then I got the deer-in-the-headlights look. I think he was maybe 22 or 23.
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