use of ago and back

Both can be used; however, using "back" in this context is informal and it generally uses a slightly different construction with implied approximation.

In your sentence I would use "ago." However, be aware that in informal speech with a less precise time period you will hear some people use "back."

For example, "A couple of years back I went to Europe." "I guess it was about five

To my ear It happened 5 years back sounds wrong, or at best non-standard.

Yes, but a lot of what I hear and say every day is non-standard.