The phrase "immature fleas" refers to baby fleas that are not fully grown or developed. They have not yet reached adulthood and are still growing and developing before they become fully mature fleas. Full definition
This residue can significantly reduce populations of immature flea stages living in the environment. (fleascience.com)
Advantage II kills adult fleas quickly within 12 hours, inhibits the development of immature flea life stages and prevents them from reaching the biting adult stage. (medi-vet.com)
The third ingredient is an insect growth regulator that prevents immature fleas from developing. (vetdepot.com)