I don't know how they would know the age, unless they just gave you a guess based on her size. 24" is pretty big though - by that age I don't know they could really give a truthful guess. That is a full grown adult! Mine certainly weren't that size at 8 months!
I've bred these 3 times over the years. None of them bulked up in the same way that say panthers or veileds will do when gravid. They do gain weight, but I haven't have one look like a big bag of marbles like sometimes the other species will. Even the day of laying and even in pretty large clutch sizes the appearance has been more subtle.
They do scout out a suitable nesting site and that is probably the best advice I can give you for knowing when (if) she(?) wants to lay- she will be restless and go to the ground and start looking- if she can't find a place she likes in the vivarium, she will try to exit the vivarium near the floor and spend a lot of time there searching for a way out. Usually they mostly avoid the floor, but sometimes come down, so that isn't the only factor- if she wants to lay she will become persistent about being down there.
My first 2 females did not hesitate to use a large shallow 16" pot with a plant in it to lay in- they only dig out shallow nests. The last female didn't want that, so when I saw here looking I moved her to a garbage can with a couple of branches, a suitable soil, and some dead leaves tossed over much of the soil for cover when she nested. Worked great.
I don't think this is a species that many people get infertile clutches regularly from, like say veileds. Whether that is because they don't have great track record for consistent longevity or whether they are less likely to cycle and need to lay in captivity, I'm not sure. I think one of the forum members here had or has a young female that laid a few infertile clutches over the course of a year, which is unusual. I am not sure but I think maybe that female was bred before she laid though- maybe not- don't remember. Maybe they will be along with more info or you could search the forums to try and find their account of it.
