I just wanted to add to this very interesting topic part of article from Plant Physiology book:
Juvenility
… there are many plants which cannot terminate vegetative phase till they reach some age, in other words when they are in their juvenility phase.
Juvenility is therefore unability of young plants to move to reproduction even if there are all external conditions met. It is well know in wood, that need particular number of seasons to start fruiting. Herbs can have juvenile phase that takes days or weeks. Juvenility also explains inefficacy of photoperiodic induction before reaching particular phase of growing. It is not measured by astronomical, but biological time. For example Perilla must have 3 pairs of leaves till it starts to sens photoperiodic induction, tobacco creates up to tens of leaves before going to reproduction. Adapt advantage of juvenility is to postpone energetically exhausting reproduction till it establishes mass of assimilation surface.
Juvenility is hereditary propriety of a plant.
Junvenile plants differ from full-grown plants by shape of leaves, longer internodes, speed of growth and easier rooting.
Junvenile and adult state are often very permanent proprieties of top meristems and tissues that they originate from. Graft from full-grown plant keeps its adulthood when grafted on top of juvenile basis. Juvenility and adulthood is kept by cloning.