An interesting subject, to me, and easily observed in many plants. For others, it is more difficult to determine. I used to think that the transition from juvenile to adult growth was fairly abrupt, happening within a year or two. But that is not necessarily so. Woody plants often take many years for the transitional phase to "complete", and in fact some never complete. The good majority of perennials, and certainly annuals, change faster and earlier.
1. Juvenile (needle like) and adult (scale like) foliage on Thuja occidentalis (Eastern Arborvitae) seedlings.
2. The same stages, and showing a pleasing variegation in adult foliage.