This is our first and we won't know the gender til birth, but I started buying little bits and pieces of gender-neutral clothes the other week, about 19+ something.
I've done a ton of window shopping online for the bigger things, and we'll probably go in a few weeks, around our birthdays, to push some prams around and make some decisions etc.
It makes sense to wait if you're finding out the gender so you don't buy more than you need as you already have boy stuff, but I reckon start whenever you're comfortable to do so, if you know what you need/want etc and it's stuff you don't want to get in gender-specific designs.
For me personally, I'm waiting a bit longer because I don't want to walk into Mothercare without a bump
I know that's just me being silly, and they're hardly going to kick someone out of a baby shop because they don't look pregnant, but I always imagined me and hubby baby shopping when I had an obvious bump, so I want to stick to that fantasy
My bump is getting there though, so soooooooon the shopping will start!