I think the oldest leaves just naturally die at some point. I noticed that one of your plants has the bulging nodes that I have a thread about here so if you would please post on there when the plant shows sex I would really appreciate it, ESPECIALLY if it's a male so I can consider that technique debunked.

BTW, if using natural light they won't flower until sometime near the autumnal equinox. They will just keep growing leaves and stems.