To turn Zombie to baby state I used:
/entitydata @e {IsBaby:1b}
or
/entitydata @e {IsBaby:1}
and it worked fine
then I tried setting it back to adult state with:
/entitydata @e {IsBaby:0b}
and
/entitydata @e {IsBaby:0}
and niether one worked
It seems to be the same for older versions too.
From @unknown: