the only down side of waiting too long is the fact that it'll get overcrowded and there isn't enough light penetration to the bud sights. Don't worry about that as it looks like you'll have enough room, seeing as they are still pretty small. Most plants double in size (depending on the strain, sativa's can triple) once you switch the lights to 12/12. Personally, I'd wait till they're about 1 foot. Here are some pics of preflowers, the males are the ones with the balls, but they can look like bananas also...sometimes early on theyr'e quite hard to distinguish. The females are the ones with the white hairs coming out.