A 4 to 6 month old would be perfect for a cage that size. I'd get a 30 high for anything under 4 months. Babies (from breeders) are normally free ranged their feeders.
Up to 4 months I use 12x18x20 so food can't get very far making it easier to hunt. After that I move them to 18x18x36. Then depending on size, I move them to 24x24x48. My outdoor enclosures for all measure 24x24x72.