My three are all >12 now, and it does seem as though we've all been getting fewer colds in the past few years.
I get about 1/2 a year I think, but I don't travel on public transport very often, and I work on my own, from home. My wife is also homebased, so it's only the kids that really bring the bugs in. We do live quite an active/outdoor lifestyle, which has to help.
Two years ago our central heating was off for the first 8 weeks of winter (plenty of heat in the playroom from the log-burner, and fairly mild), and none of us got a cold. heating back on, and then the colds came..