Humans always try to categorize things, events, eras, etc, because that's how our minds work. And then we like to think the divisions between the categories are logical, but mostly they're based more on emotion. I think it's easy to argue that the advent of brifters was a watershed, and recent enough that the bikes are merely OLD rather than Vintage, but drawing the other line when 10 speeds became available? Much murkier. There was no technological innovation that suddenly allowed Shimano and Campagnolo to suddenly put another cog on there. They just made them thinner with less space between them and added another click in the shift mechanism. Same with 11, 12, and now 13 speeds. Has there even been a similar watershed moment since brifters?

I find it interesting that when the Eroica rides started in 1997, they just went back 10 years to 1987 for the division. But then rather than sticking with "> 10 years old" as the definition, they stuck with "Older than 1987". So, every year, the bikes ridden there are another year older.