Those who are kept in your inbox or any other folder need to be removed manually unless your webmail client or desktop client has options to do that, which is likely to happen, just search those emails and then delete. You cannot do that from the Mail app though, and not sure if you could do it from a third party app.
If you are using iCloud accounts, go to Settings > Mail, Contact, Calendars > tap in your account > Advanced > Mail > Advanced > Deleted messages > set it to "remove after one month" or whatever other option, but they need to be marked as deleted anyways.
And last but not least, you can create a rule in your email server so all emails older than 1 year, go directly to trash, or even delete them, but again, not in the iOS Mail app.
https://support.apple.com/en-us/HT201419