How old a version can I upgrade from?

Any version. Even if 1.0 is installed its ok to upgrade to the latest version.

Actually its also ok to downgrade. The database evolved by 'adding only', making it possible to move up or down versions. For example, if you upgrade to 6.0 and don't like it for some reason, you can downgrade to 5.9. Then if you want to try 6.1, you can go from 5.9 to 6.1.