Baby
From Tardis Wiki, the free Doctor Who reference
Babies are what humans call their young. In general, human offspring are referred to as babies from birth to about when they start to walk. According to the Eleventh Doctor, babies had their own language, which he could speak.
The fleshkind baby, Sky Smith, was primed to rapidly age into a pre-teen upon encounter with the metalkind. (SJA: Sky)