In the Proclaimers Book it says this which seems to indicate that they stopped celebrating birthdays when they were still called Bible Students which would have been prior to 1931 when Rutherford renamed them "Jehovah's Witnesses."
This is another porky from the WBTS as evidenced by the letter from a faithful Witness in Germany in 1944, and my own experience of faithful Witnesses still celebrating birthdays until 1951.
BTW Doug, from where did you get that quote?
yesidid