I've heard that you can inspect the vent to potentially check for laying ability. For older hens, if the vent is moist and pink then they are still a good layer, while if it is dry, cracked or white they are probably not laying much if at all. I don't think this method works for aging per se, but it will tell you whether the hen is still laying.
As for hens that are still young and may or may not be laying, I would look at the color of their comb. If it is lighter pink or whitish, they do not have the appropriate levels of hormones yet to be laying. A bright red comb in a younger hen is a sure sign they are laying or will be any day. Good luck!