No, baby spirits do not age, just as no spirits age. Once they have died from mortal form and become a spirit they are no longer subject to aging & death. They remain in the state they were when they died; especially with regards to their mental capacity. They are spirits, therefore they have more ability than we mortals, but they aren’t as experienced, nor as developed, as those who lived a full life span and died. Adolescent and youthful spirits are distinctly different than adult spirits, and they can be more of a handful to care for because of their non-developed state.