it's a dilineation that does not matter much...
it's not a 'one day it will happen overnight!" type of things... some may be more robust and mature before others... you can tell if they can handle 35-40DLI without shriveling up.
Vegetative growth -- new leaves and stem elongation.
bloom -- growth related to sex organs and cessation of vegetative growth.
There's no definitely point where a plant is no longer a seedling or immature plant... it's just about how it behaves and some plants will have different timeframes.