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As soon as seedlings sprout, the seedling stage begins.
After a couple of weeks you are probably ready for transplanting into larger pots, this is when you start to feed nutes to the seedling to encourage it to grow into the vegetative stage, where the stems and leaves grow much faster
As you know, for example, when you give the 12/12 and force the vegetative plants into flower, they usually take about a couple of weeks to really act flowering.. so my opinion is that seedling is just a transitional fase between the seed sprouting and the vegetative state, in which the plant is very fragile.
I'm sure some more botanicaly inclined person will give you a better answer though
Personally, I think it's a seedling until the second or third set of true leaves. Then it's goes to the veg stage. It's how it works in my hollow head, anyways.
Seedlings are the stage the plant uses to develop the tools it needs to survive. That's why you trim leaves on clones and put them in low light with no nutes. A cutting will kill itself trying to process food, water and light with no roots.