minhag - Source for Tehillim of your age?

Does anybody know of a source for the minhag to recite daily the perek of tehillim corresponding to your age?

Answer

This is a custom transmitted from the Maggid of Mezritch to the Alter Rebbe of Lubavitch in the name of the Ba'al Shem Tov.

This is recorded in a letter of the Previous Lubavitcher Rebbe, printed in the back of תהלים אהל יוסף יצחק. In this print it is on page 214.

The full letter is printed in Vol 10 of his letters, page 53.

The reason for the custom is cryptically described by the Alter Rebbe here (מאמרי אדה"ז הקצרים page 341), which is somewhat explained by the most recent Lubavitcher Rebbe here.