Select any email list and, where you have an accurate count (eg 1 – 50 of 500), hover on the count to see a drop down showing the options Newest and Oldest. This will sort the email in that list by date – newest first, or oldest first.

Where there is only a estimate (eg 1 -50 of many) it is an indication that GMail has not yet counted all your email in the chosen list and the sort option will be greyed out.