Delete photos older than x days in iOS
Sure - if you don't need to make backups or transfer, install the Shortcuts app (used to be called Workflow) before Apple bought the company and renamed / shipped / enhanced the automation app for iOS.

You need two items - select photos and delete photos

For select, you can choose photos before a date. If you're not sure, you can limit it to 5 photos so you can validate it's getting the ones you want to delete.

Just be sure you know if your iCloud photo library is tied to the iPhone since deleting a photo will then cascade and delete in the cloud as well.


Delete photos on iPad without affecting photos on iPhone
Deleting photos off of my iPhone but leave photos in the cloud