Simply replace the unfilled areas such as "__" and "_[date]_" with your information.
After several years of wondering where life had taken you, I finally decided to do something about it. I tracked down your information by _[how information was obtained]_. I was so thrilled to finally locate you and I couldn't wait to write you.
I hope this letter finds you well. How excited I was to find you living in _[location]_. Do you enjoy living there? I hear it is _[adjective]_. Are you still a _[hobby or profession]_? It would be so nice to hear all about where life has taken you.
Let me tell you a little about my life these days. I am living in _[location]_ where I am a _[profession]_. I am currently _[specific current activity]_. In my spare time, I _[current hobby]_. Other than that, I am pretty much the same _[guy/girl]_ you once knew.
I can't wait to hear from you.