[Wow, forgive him if he doesn't believe a word of that. And he didn't ask for advice from a young traitor-to-be anyway.]

It's unlikely you'll remain here much longer. However long you stayed here, your change in age was unnoticeable when you returned. [Even through the use of transformation jutsu, he would doubt more than a year or two's difference.] With such uncertainty, time put into training these creatures is ultimately wasted.