I have heared about this, were your tamas differant versions? like V4 and V2 for example? This happens because when you connect with a older version your tama will not show up on the old tamas screen as your character but as Nazotchi <_< This happens because the older version isn't programmed for the new characters, therefor it shows up as Nazotchi!
And Nazotchi is classed as a adult, so the older version will think your new tama is a adult when it may not be. And the teenager should leave after a few days!!! And i doubt it will transform into a adult!
Hope i helped, and have a super tama day!