I have never really thought about the lowest temperature for them, any more than I have thought about the maximum temperature. Budgies, Lovebirds and Cockatiels originate in Australia and Africa where the extreme temperatures are more in the high temperatures than the low ones. They have feathers which provide insulation, but their feet have no protection. If it feels cold to your hands (you put your hands in your pocket or put gloves on your hands) if you are outside for a few minutes, I would say it is too cold for them to be outside without a heater. If you use a heater, please be careful that it does not have teflon.
I am just on my way to work in a few minutes and may re-word this to make more sense after I get home.