There are indeed baby spirits.
Like all spirits, they neither continue to age nor die after transitioning to spirit form.
Spirits do, however, retain their “cognitive state personality”. This means that when a being dies at a young age and transitions into a spirit, they retain their youthful persona.
Young spirits are highly evolved, but since they never experienced the full breadth of life, they lack the depth of wisdom that only comes from the full living-aging process. This presents them as innocent, naïve, trusting, and playful, which are some of their most endearing qualities!
Keepers enjoy the experience offered by young spirit companions, which is quite different than that of adult spirits. They especially appreciate the strong feeling of being needed, which is much more pronounced when keeping young spirits.