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.