Treat Her Gently - Lonely Old People is written in the key of D Major.
According to the Theorytab database, it is the 2nd most popular key among Major keys and the 2nd most popular among all keys. Major keys, along with minor keys, are a common choice for popular songs. The three most important chords, built off the 1st, 4th and 5th scale degrees are all major chords (D Major, G Major, and A Major).
See the D Major Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!