lasauge 

I lived without a car in Colorado Springs, a city that's less bike-friendly than Boulder or Fort Collins for a decade, so it can definitely be done there if you're determined enough. The two things I'd add to this thread are that the winter weather in Colorado is relatively mild compared to what much of the country gets, but you still need to be willing to don cold weather gear and be willing to accept the challenges of riding on ice and snow on occasion if you want to ride all year round in this state. Bike paths do get plowed and sanded, but plows can't be everywhere all the time, and side streets that are often the best routes for cyclists also naturally are a lower priority for plowing, so plowing might not be complete everywhere along your route. Also, it doesn't happen very often along the Front Range, but there are always a handful of days per year where there's enough snow that riding is going to be physically impossible (in which case the city will be half shut down anyways) or be challenging enough to qualify as a winter adventure sport, so if that's not something you're okay with then keep in mind that you'll want to locate yourself carefully to have good access to transit. Second, the paths are great in Boulder and FoCo (and the Springs too, albeit to a lesser degree) but they can't go everywhere, so as a practical matter it's tough to live car free here if you're not willing to ride anywhere but low stress routes, so depending on your tolerance for stress you might be better served prioritizing a place with more density and better transit.

As a more general point my advice would be to figure out roughly where you want to be for family, work, recreation, etc. first and then explore options for living in that general area. Living car free can be done just about anywhere with the right adaptations if that's your priority.