There is no way that I know off to change the sort order on the iPad. The most recently received emails are on the top.
Depending on your email provider, you may be able to do something about it on your webmail site and it's settings.
Out of curiosity, are these old emails showing old dates in the emails header information (under the email title)?
Yes the 'sent' date is the old date but the 'received' date is current