How do I delete files older than N days?
Sure - a safe way is to have a backup like Time Machine - you can then delete files knowing you can restore them with ease.

I like to sort a file listing by date - select the first file to be deleted, scroll to the end of the list leaving that one file selected. Then use shift to select the last file - shift then selects the entire range.

Then put all the files in trash. Once you're sure your backup was was good or you selected the correct files - empty the trash.