According to Microsoft, development on Age of Empires has ceased to be profitable, and will therefore no longer be done. On his blog, Executive Producer David Perry was quick to reassure people that the game is still running and there is no cause for worry; the only difference is that there will be no new content from this day forwards.
Publisher Microsoft and developer Gas Powered Games will continue providing support for the game, but Perry conceded that:
Fixing any remaining bugs and addressing balance issues will be slower and, frankly, more difficult for the team. Some may, in fact, be unfixable. We will be watching carefully in case any critical bugs appear.
Age of Empires Online is one of Microsoft’s few attempts at free-to-play. The game was initially criticized for being too miserly with its content, though subsequent updates lowered the costs dramatically and the game went fully free-to-play in June 2012.
You can read the blog post here.