This may sound like a silly question, but I have never been in this situation before and therefore have no experience.  I live in the Northeast and have a litter on the ground.  My question is as follows:  At what age can you start introducing puppies to brief periods of cooler than room temperature?   My past litters have been out on the ground by 6 weeks of age on nice sunny days with temp above 60.  Currently I have frozen ground or wet and near frozen.  If I can find frozen dry ground at what age might it be safe to let them run?

Personally I would let them out around six weeks and keep a close eye on them.  If they are romping then fine, but shivering and standing around looking forlorn, time to head back in.   Just use common sense and you should be fine.

Ken

Ditto.  I had a litter whelp near Christmas, and the pups' first trip out was at +7(F).  It was very breif. Common sense, and your own thermal comfort will let you know.  Remember that there's a huge difference between 55 and sunny and 55, cloudy and windy.