My personal threshold is low 30s. Today is in the low 40s and I am going out riding for sure.

As far as layering, I go with Merino wool base layer, jersey (short or long, depends) and windbreaker or wind vest. On the bottom, I go with bike shorts under tights. Merino wool socks, and shoe covers. Long fingered gloves. If below 45 or so, merino wool gloves. Above that, I can usually just go with long fingered mountain biking gloves.