I know that both of the reptile vets I see regularly (including board-certified reptile specialist Dr. Adolf Maas DVM, DABVP; one of only twelve such
certified specialists worldwide) strongly discourage popping anything older than one month. Dr. Maas in particular told me about a couple of cases he's seen with male snakes that were seriously injured by popping. Maybe someone with sufficient experience could do it and the vets are referring to recommendations for inexperienced owners; I wouldn't know for sure because I haven't asked them. Either way, I'm only relaying their recommendations because I want to help ensure that he stays happy and healthy, that's all.
For anyone who cares to check (or needs to find a reptile vet in the area), the second reptile vet I mentioned is Dr. Elizabeth Kamaka, DVM. She was my teacher for exotic animals during my training as a vet tech, and I consider her a personal friend. I take my snakes to see Dr. Maas, because his practice is specially set up for routine, preventative, and emergency and long-term care for exotic animals of all kinds and because his practice is open every day while Dr. Kamaka's practice is only open three days a week. Both are excellent veterinarians who know what they're doing with these animals, are affordable, and have great personalities. The other vets working for Dr. Maas are just as good, as well.
Dr. Maas runs the Center for Bird and Exotic Animal Medicine in Bothell, WA
Dr. Kamaka runs Kamaka Exotic Animal Veterinary Services in Mountlake Terrace, WA
I understand that ACCES in Seattle also sees exotics, but they're more of a "companion animal" specialty center and I honestly trust Dr Maas more.