This is a very individualistic thing and a lot depends on the climate to which you are accustomed. NW climate is moderate with very few, but short lived extremes. Extremes to us are lows in the teens and highs in the 90s. And our humidity is generally quite low compared to the rest of the country.
So riding in high 30s is tolerable. Of course, anything warmer is better. My ideal is 65* - warm when off the bike, a cool breeze when under way.