There's your answer then. The older hens don't go up because they can't see the roost after the light goes out. And if the light stays on, there's "drama" because the older hens are trying to exert their dominance and kick the young ones off the roost and/or claim the best spots. Why do you need to have a light on? It's summer, it's bright enough without a light. Keep the light off permanently and let their eyes adjust naturally as the sun sets and it gets dimmer and dimmer. If you've had a bright light on and it suddenly turns off, the difference between light and dark is sudden and exaggerated and your older hens are suddenly blind. If they have time to adjust to the natural dimming of sunlight, they can time their roosting better and start getting up there earlier, as opposed to having a bright light thinking they have more time to eat and mill about, waiting for the "sun" to set. Keep the light off and let them figure it out.
