If you keep a busy schedule or you don't normally regulate iPhone storage, iPhone messages begin to accumulate and occupy too much space on your device. It's a pain to incrementally delete each message, so thankfully, there's a setting where you can automatically have your iPhone delete messages chronologically.
How to Automatically Delete Old Messages
Open Settings > Messages
Scroll down to Message History
Tap Keep Messages
There are three options: Forever, 1 year, and 30 days.
Once you select 1 year or 30 days, any messages older than that will be deleted.