Experience Points are obtained by upgrading buildings and/or troops. Levels are gained by reaching a specific amount of XP. To find out how much you need, you can tap the info and level bar in the top right of your screen. Back to Levels, you also need to be on at least one level that the game says. To find out which one, tap on the info part of your Town Center and if there isn't a level counter then you are able to carry on to the next Age.