What is an old-growth forest?

Question

What are the characteristics that make the average forest "old-growth"?

Answer 1

Old growth forests are forests whose natural cycles of growth have not been disturbed by logging, building roads or clearing.
Other words for old growth forest are 'original' or 'ancient' forest. A forest is a living system - ongoing cycles of birth and death, growth and decay. It may appear as if nothing is changing but countless natural cycles are at work every day and night. What is unique about old growth forests is that those cycles have continued uninterrupted over a very long time.

Answer 2

An old-growth forest — also termed primary forest, virgin forest, primeval forest, late seral forest, or (in Britain) ancient woodland — is a forest that has attained great age without significant disturbance and thereby exhibits unique ecological features and might be classified as a climax community.