I think it depends not only on development but on the room they have to continue to grow.
Like Hendi_Alex, I've got several seeds in that 1st container. In fact, I usually have dozens together! Once they have decent true leaves, they can't grow much more without serious crowding.
Someone once told me that a plant usually has quite a few leaves and quite a few roots -- but, often only 1 stem. It might be the sturdiest part of the seedling but I try to not even touch
that stem with my big, clumsy fingers! A seed leaf is what I prefer to hold onto, if possible - as I pop the plant out of soil with a popsicle stick, or something similar