1. Next to the Search the Web button up top, click Create a filter.
2. In the "Has the words" field, type before:2015/01/01 (just an example date) it would delete all messages received prior to January 1 of last year. You can use any date you want, as long as it conforms to the format YYYY/MM/DD.
3. Click Next step.
4. Check the box marked Delete it, then check the box for Also apply filter to ### conversations below. (Tip: If you want to preserve your mail while still getting it out of your inbox, choose Skip the inbox (Archive it) instead.)
5. Finally, click Create Filter. Depending on how many messages meet the filter criteria, the deletion process may take a minute or two.