23+ years, as most of my bicycles seem to be from the mid-1980s, although I also have a 2012 Raleigh Sojourn tourer, and a newish Diamondback hybrid. When I got these bicycles, it was because I was the only person interested in them at the time.
Average is 7 years over 5 bikes. The oldest being 16 years old and these days relegated to being permanently attached to the smart trainer. And the youngest being 0 years old as I'm just finishing it as a complete new build and which should get its first ride Sunday.
This is getting competitive. My Surly Steamroller, frame new and built up 2007 . Since then almost every thing changed including the frame from 62cm to 59 cm . Only original parts front brake caliper and one of the bottle cages . mmm maybe it’s a new bike with an old front brake and bottle cage attached.
My fixed-wheel gives me a problem. I put it together in the early 80s, but it has had a lot of replacement components, including frames, since then. Even more complicated, one frame (not the current one) was made around 1960. So I could legitimately date it anywhere between 5 and 60 years old.