I'm an outsider to this, so I can't tell if people are referencing mass start participation numbers or TT, but... maybe 45 is the sweet spot. Any older and it's too old for sports. Any younger and they'd rather the appeal of a mass start.
So... I went and dug around USAC to find some comparable stuff and they were consistent with your proportions. These first three illustrate it, however the bottom two links suggest a more tapered distribution. But for the most part, it looks like that's a typical distribution from the few I reviewed.
http://www.usacycling.org/results/?permit=2011-2265
http://www.usacycling.org/results/?permit=2011-2297
http://www.usacycling.org/results/?permit=2011-384
http://www.usacycling.org/results/?y...&info_id=39959
http://www.billboneproam.com/Event-Results.aspx
I suspect time trial only events are skewed differently than multi-format events/omniums.