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.
Comments
Re: Juvenile and Adult Phases in Plants
I know that some cultivars of Thuja have the juvenile foliage all life and some junipers show both juvenile and mature foliage.
The juvenile leaves of the holliy (Ilex aquifolium) are much more spiny than the mature ones, at least of the wild type. If you cut it back it produces very spiny juvenile foliage, certainly against foraging animals.
Re: Juvenile and Adult Phases in Plants
Steve - there was I hoping you had pictures of the ten foot tall chickens! We had several Pseudopanax in the garden until the last year or two's very severe winters. They must have been reaching the height where they begin to change growth habit. Really quite surreal plants.
Re: Juvenile and Adult Phases in Plants
I guess it must be a tasty tree. My immediate though was that it is the quickest way to reach for sunlight in a dark forest.
Indeed, many conifer cultivars are selected for juvenile foliage. These are in my backyard: Chamaecyparis pisifera 'Squarrosa Lutea'.
Thuja occidentalis 'Heather Bun' - on the left. (30 November 2011)
Re: Juvenile and Adult Phases in Plants
I know that some cultivars of Thuja have the juvenile foliage all life...
Speaking of that, here's one of them... Thuja occidentalis 'Teddy': 
Apparently, it gets to a foot tall at maturity. I picked it and a small number of other rock-garden trees up this summer - no idea how it will do (other than that arborvitae in general tend to suffer here in the dryness). Has anyone around here grown it?
Rick we have a lot of trees and shrubs with juvenile foliage in New Zealand . They assume this was to combat browsing by 10 foot tall chickens (moa) . I don't have many pictures but there is some reading available at http://www.teara.govt.nz/en/shrubs-and-small-trees-of-the-forest/4