Able to roost and will roost are two different things. I've had a broody hen take her 2 week old chicks to the roosts, so they are able to roost that young. It was really warm weather so that played into it since she could not cover all of them on the roosts. I've had some broodies wait until 4 weeks old to take her chicks to the roosts. I'm not talking about perching and playing on perches during the day but actually spending the night on the roosts.
I have had brooder raised chicks start roosting as early as 5 weeks, but that is pretty unusual. Move average for my brooder raised chicks to start spending the night on the roosts is around 10 to 12 weeks, but occasionally some will wait even later.
I'm not sure exactly what your great aunt is talking about. She might be talking about them being old enough to roost without getting too cold, she may be talking about brooder raised or broody raised, or she may be talking about something else entirely. They are able to perch at an extremely young age, but they do need some age before they sleep on the roosts at night. But not a whole lot of age depending oin the weather.