You can use folder settings to automatically delete older emails in a folder. For example you could automatically delete the trash or spam folder after 2 weeks or 1 month.

Tap on the "Folders" button in the left side panel. Then tap on the folder button beside an email account to show the folders. You may have to tap the left arrow beside the folder heading to go back to the correct account.

In the top of the folders list, tap on the settings (cog) button to edit the folders.

Tap to select the folder you want. In the top of the folders list screen, tap on the action menu (circle with 3 dots button) to view the actions.
Use the menu action 'Folder Settings' to edit folder settings. You may need to look under the 'More' menu to find it.

In the 'Folder Settings' screen you can turn on 'Delete emails after (days)' and select the number of days you want to use.
Older emails will be deleted when the folder is refreshed.