AirTags will work with older iPhones but things are a little different.
The trick is that they work by Bluetooth with a range of about 20 to 30 feet. The last known location showing in Find My tells you roughly where they are. The next step is to try activating the sound feature. If you see that it's working you know the lost AirTag is within about 20 to 30 feet. If it doesn't work keep moving until you start to see it working. Now you can try to listen for the sound. It can be faint. It's also short-lived. You may have to keep tapping the sound icon. Eventually you'll find the missing AirTag.