The internal format of toolboxes can change from release to release. In all cases, newer releases (such as release 10.2.1) can read older version toolboxes (such as 10.0) and execute their tools. However, the opposite is not true—ArcGIS 10.0 cannot read toolboxes created in release 10.2.1. If you are working in an environment where your colleagues or clients have older versions of ArcGIS, you can save toolboxes to an older version and deliver that toolbox.
To save a toolbox to an earlier version, right-click the toolbox and click Save As, then choose the version.