i personally dont think the size vs age matters as its dependant on how much the ray eats or is fed . some rays will eat more in some circumstances and if you can get them to eat enough they develop a large bump to show it.
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ive had pus from the same parents born at a very different size. 3" - 6". i dont think this reflects the adult size. just how well they are doing and how much they have been fed up to that point.
that said. you could try underfeeding a ray to try keep it small but thats not a route i would like to take.
i got rid of a 22" female marble some time ago and i have a large female still. however most of my rays are small and from small adult parents.