If your local mail is stored in single files like:
/var/spool/mail/user
Then you can use the mutt command to delete emails older than x number of days or months.
Eg

# mutt -f /var/spool/mail/luke

Once mutt has loaded, use the following set of commands, replacing 30d with the emails you with to delete older than
Eg I with to delete emails older than 30 days:

^D
~d >30d

You could also used m for months and y for years