apple mail

OK, should be easy, in Mail's tiny header, click on Date Recv'd to sort by age...

Scroll down to the date you want to start deletion at, scroll down to end then hold Shift wilst selecting the bottom one, that should highlight a whole range of eMails, Use CMD+Delete keys to delete them.

Alex, is it an IMAP account?

Do you want to delete everything in Mail?

Older eMails or older Accounts?