I went with 5-6 months. My kids were born in Aug and Sept. so by the start of the next season they were about 6 months old. I don't think it matters though. You could porbably take a new born out as long as you keep the sun of them and it's not to hot. One more thing, I don't think you are supposed to put sunscreen on a baby for the first six month.
Here's a pic of my son having a "cool one"