Really depends on how you are dressed, and your ability to stay warm and dry. We often sail with air temps in the low 20s.... its fun and safe as long as you stay warm. Get wet and or cold, and suddenly its dangerous.My son came up for a visit and we wanted to get him out on the boat.
We arrived at the marina and it was 19f out, so we hung out for a couple hours to see if the sun was going to break through and get the temp up to the predicted 50f. At noon we made it all the way up to 20f..... yup, we left without a sail. Pretty sure that was the right call that day.
That makes it worse!!!clothing was not the issue. It was just a bit cold and foggy, had the sun broke out we would have went. With it being his first time sailing I'd rather have it be a bit nicer for him.