Season? What is this thing you call season?
For me it depends on the nature of the ride. Having been car-free since 1999, for commuting or errands, I haven't yet found a lower limit on temperature, or a depth of snow that keeps me indoors--much to the consternation of my driving co-workers. They've learned not to complain about conditions on their commutes.
For the record, I'm very, very seldom the first tracks out there either.
When it comes to purely recreational rides, somewhere in the teens (F) is my limit. It all depends on how itchy I've become and how nice the day is otherwise. Although we used to have a weekly group ride through standard time that met every Tuesday evening. Before my hours changed at work, I was a regular at that, no matter the weather.
I am not worthy!
Being born and raised in SoCal has made me a weather wimp. First, I would not ride my bike in the rain unless it was an all out emergency. Second, riding when the weather drops below 45 is not an option.
I am not worthy!