A market is mature when...
This tweet reminded me how competitive app stores are. Mobile apps were once a brand new market with lots of opportunities and space for innovation. They're not anymore, as you can tell by this picture.
A market is mature when it has reached a state of equilibrium. A market is considered to be in a state of equilibrium when there is an absence of significant growth, or a lack of innovation. When supply matches demand the price decided by the market forces of demand and supply is called equilibrium price. (Wikipedia)