You're only setting the API level with which you are compiling your app. This should always be the latest SDK version. This won't affect with which minimum SDK version you can run your app.
You have to set the minimum SDK version in your Android Manifest:
<uses-sdk android:minSdkVersion="18">
It's API 18 in this case, but I assume you want to support older API's as well. Most people either only support ICS+ (API 14+) or go from down to GB (API 10+) all the way up.