Well, first of all, you really don't know if your young mimitchi is going to turn into a mimitchi. Mine wasn't a young mimitchi teen, but it ended up turning into a mimitchi adult. Anyway, again, you must receive high intelligence points (high as higher than Social (flower) and Arts (sparkle) points), because of the fact that in the Tamagotchi Profiles sheet given out from the V4 package (if you're talking about V4), it tells that the Mimitchi is smart, so it probably should have high intelligence. About how long it will take, I'm not sure. But, it will happen soon enough- it only ages when it is sleeping (but it does not evolve during sleeping) and when it is being played with (off pause).