The wiki states that in Extreme Hills biomes, snow falls above 95m, because they have a temperature of 0.2. Given that Taiga biomes have a temperature of 0.25, at what height does snow fall and water freeze?
Looks like it might be a gradual change from not freezing at 122 to freezing pretty quickly at 127 or so. In the image, the large pool on the right is at 123, then they step upwards to the left up to 127 at the far left. At the lower right, the pools are at 122, 121, and 120. They have no ice, and the ones I made most recently, the highest ones, froze over the most quickly.
Just realized your question was mostly about snowfall; this will occur at the same height that water freezes at.
Edit: After over an hour, this is how much has frozen. I recommend building ice trays and the like starting at block 130 or so, just to make sure you get quick and complete freezing.
Please note the water pools are of different dimensions, which skewed your results. Water in Minecraft freezes only if next to a solid block. The smaller pools in your test had many more edges per water block compared to the larger ones. If the test were conducted properly, you should find the ice forming at roughly the same rate from height 123 upwards.
I had som weird experiences with my Etho style Ice Farm and had to research this further.
My farm was initially built with the water source blocks at height level 124. And as you can tell from the picture above my farm was consistently producing ice on only some of the water blocks, giving a rather slow rate and yield.
I first thought this was just due to the randomness algorithm being buggy, but the real explanation revealed itself once it started raining:
It seems snowfall is very sporadic all the way up to height level 137. Only at height 138 was there no sign of rain around me:
I will try to rebuild my farm so that the water source blocks are at this height and report back with the results.