That was my conclusion after watching it for my first time. As for the kiddies being aged and grey, I'd say it's more like they're really old, but kept at a child-like state thus keeping them to sound like kids, around the same height too, and remain somewhat child-like. Psychic stuff and all I gander. As to why they're not like advlts, well, I'd say if they were , they'd make too many independent choices, that are likely to svck for everyone, but ultimately the military. Case in point, Tetsuo. The military guy didn't want the can of worms opened immediately, and gradually step him up. The scientist guy ignored it and went to 11 immediately and bad things happened due to his really bad inferiority complex. I'm guessing the three were kept in the child-like state as a way to control them.
It's not really made clear in the film, but it's really more the drugs and experiments they were subjected to, which implicitly stunted their growth while rapidly accelerating their aging (or...aging normally, whatever), paradoxical as that sounds. Said drugs, which awaken and enhance powers, but keep them from reaching their full potential, are also highly addictive, which helps keep the Espers on a leash. The cult leader we see a couple of times in the movie is a much more important character in the manga, and was in fact an escapee from the experiments, and since had some more natural growth. She also took some similar children under her wing who are not dependent on the drugs, but consequently are also weaker than Masaru, Kyoko, and Takashi.