Gmail is great, with nearly 8gb of storage there really is no need to delete old emails, instead just archive them and then when you need to find an email from 4 years ago you can find it via the powerful search engine.
However I have a collection of emails which are auto filtered from my webserver letting me know that the firewall has blocked access to an IP or that I’ve logged in with the root password. Whilst useful at the time, not really need for more than a month, let alone years. You might have the same with a discussion board, or freecycle.
So the quick way to delete them all:
In the search box enter
Once loaded you should be able to click on the select all button (although it’s no longer labelled).
Just above the first email you should now see “All 20 conversations on this page are selected. Select all conversations that match this search”. Click on the link.
You can now click on the Trash/Delete button.
All the old emails have now been trashed.
For the search field the date is in YEAR/MONTH/DAY format, instead of label: you can use from: so
from:email@example.com older:2010/06/12 for all emails from Lincoln Freecycle older than a year.