Don't laugh, but I start using my heated gear when it gets into the 60's. It's been in the 40's and 50's this week so it's definately in full use.
Ha! I don't own any heated gear and I'll ride as long as it's 38+. Main reason I won't ride when it's colder than that: very good chance of ice!
On the other hand.. I'm probably die in Vegas heat in the Summer!