Mine has moved from "whatever I could afford" to a nice, well-rounded collection of bikes that not only fit me (what a concept!) but that each fulfill specific purposes. They've come pretty much full circle- I am in the process of weeding out the last couple stragglers I don't ride. One is that Allez I bought recently that didn't fit- group is going on the mixte city bike build (which will fill the specific purpose of being a cute mixte bike) and frame will be sold, or built into a single speed and sold. The second is a barn find I've been stuck on for awhile- it needs to get overhauled and moved along, but the bottom bracket is so shot that I can't even take it apart. The last one I was torn about for quite awhile: it was the first bike I had built for myself, from scratch, and it had taught me a lot about bikes. But the reality was that it didn't ride well; it's worth far more in sentimental value. That one is also in the process of being dismantled. Wheels will go on the mixte, parts will go in a box, and the frame will sit until I figure out what to do with it, but I can't imagine selling it. I painted it myself, I spent hours building it and rode it for a year and a half, and selling it would mean haggling with somebody to get probably $25 for it.
Now I've got road covered (modern and vintage), mountain (again, modern and vintage, both Specialized, the latter in the form of a city bike), and town/cruiser (the Electra cruiser and, soon, the mixte). As far as decades I seem to stick to, the oldest bike is only a '92 or '93, if we're counting only my permanent collection. Though I've seen a lot of bikes I love from the '50s, '60s and '80s especially, finding them in my size at a reasonable price is dang near impossible. I like my collection to represent that I have an appreciation for vintage bikes, but ultimately, like with most people on here, these are all riders.