Outstanding! It’s great to hear that your 16-week old Hippocampus erectus juveniles are continuing to thrive and that some of them are beginning to develop incipient pouches.
No doubt about it, raising a large brood of seahorse fry, or several broods, is a Herculean task that is going to stretch the time, resources and patience of the home aquarist to the breaking point, and it’s very gratifying to hear that all of your hard work and effort has been rewarded.
It sounds like your youngsters are right on schedule with regard to their sexual development, Tammy. Breeders report that captive-bred-and-raised Hippocampus erectus may reach sexual maturity within 4 months of age at a size of about 78mm or approximately 3 inches in length (Vari 1982). But it’s hard to put a time limit on how long it may take for all of your juveniles to become sexually mature, Tammy. There is often considerable variation within a brood of young in that regard — some precocious ponies begin to show pouch development at the tender age of 3-4 months while at the opposite end of the spectrum there may be a few late bloomers that may not develop fully functional pouches until they’re yearlings. (It’s a lot like puberty in humans; some youngsters begin to develop while they’re still in elementary school but others don’t hit puberty until after high school. Most are somewhere in between.)
You’re probably going to start noticing that some of your young adults are beginning to pair off soon, and it won’t be long before some of the 16-week olds are ready to begin producing offspring of their own. So you may want to start separating the young stallions as they emerge if you want to prevent any unauthorized breeding among the youngsters at this time.
Best of luck with your precocious ponies, Tammy! And please accept all my condolences over the loss of your father.