The ages shown in those photos of the birds whose genders are in question is when the shiny feathers should start coming in. I can't remember the weeks exactly, but I'd definitely say it is beyond 4 weeks, probably beyond 5 weeks. It's pretty much when they are all feathered in.
Bantam and LF standards are the same far as I remember, but I'm not much of a bantam researcher honestly. Answering on the bantam flock. I say get yourself the Standard of Perfection, weigh your birds, and see who's closest to the weight. They shouldn't be small as you can go, no, they should be just right. If you've got birds over and/or under, you might want to calculate every other aspect of theirs and see if the farthest from the standard bird also has the worst weight. See what options are out there for replacement, or if you want to do so yet.