I'm not sure Odo is considered 'fully developed' by other changelings. His capabilities aren't (given that he still can't mimic human faces well - or is that something he consciously chooses to do, remembering his early days? Can't remember whether that's a fan theory or something actually said in the show), and both the female Founder and Laas (who never was in contact with the founders) treat him as someone with only a limited amount of life experience, much like a juvenile. But yeah, he's certainly further along than an infant Changeling. I don't think so. I only recall them saying they weren't expecting anyone back for another 300 years.