The game will run you through the tutorial to upgrade a few buildings and eventually, when your ready, you will have the availability to upgrade your keep. Upgrade your keep 7 more times to reach the Eight Age.
I would recommend that you feel fairly comfortable and stable in your current age before upgrading. Ensure that whatever troops, defensive items, and whatever else you use are fully upgraded to their respective age. If you progress too quickly, you’re likely to get slaughtered by your opponents.