Whenever you're ready. My toddler had rain gear (a full suit and rain boots) and I plan to get the same for my 3 month old whenever she starts walking My little baby takes showers so I can't imagine she'd be bothered by the rain. We'll be spending this winter somewhere with snow so hopefully she'll get that too. I say early is best!
i live in washington and baby wear most of the time when we go outside so she’s been exposed to rain, wind, sun (with a hat and sunglasses). all i’ve really heard is make sure they’re dressed appropriately, no direct sunlight if possible.