Does iPhone 7 is comparable to Airtag?

No they are not comparable, one is a phone, the other is a locating device.

Do you mean what phones they are compatable with? They will work on any phone running iOS 14.5 or higher, so yes, they will work on an iPhone 7

AirTag - Apple

Your phone is not too old. You phone is capable of running iOS 15.6, the minimum requirement for Airtags is 14.5 so you meet that requirement.

You can use Airtags with your iPhone 7.

You're welcome.