From the founding of the Bible Students they celebrated birthdays. Here is an article on the history of birthdays in the WTS.
http://www.jwfacts.com/watchtower/celebrations.php
Birthdays
The following quote from the 1975 Yearbook makes it appear that birthdays stopped being celebrated shortly after Christmas in 1928.
"After that, the brothers at Bethel never celebrated Christmas again. "Did we mind putting those pagan things away?" asks Charles John Brandlein. "Absolutely not. This was just complying with new things learned, and we had never known before they were pagan. It was just like taking a soiled garment off and throwing it away." Next, birthday celebrations and Mother's Day were discarded-more creature worship. Sister Lilian Kammerud recalls: "How readily the brothers all dropped these holidays and admitted they were glad to be free. New truths always make us happy and . . . we felt we were privileged to know things that others were ignorant about.""Yearbook 1975 p.147
God's Kingdom Rules! makes it appear that birthdays were identified as unacceptable in 1936, by quoting from The Golden Age of that year.
""There are but two such celebrations mentioned in the Scriptures, one of the heathen king Pharaoh of Egypt, in the days of Joseph, and the other of [Herod,] whose birthday cost John the Baptist his life. In the Bible there is no instance of celebrations of birthdays by any of God’s people."—The Golden Age, May 6, 1936, page 499." God's Kingdom Rules! (2014) p.105
In reality, birthdays continued being celebrated until 1951. In 1940, the following experience shows birthdays were still considered appropriate.
"Pardon me for intruding on your precious time, but I just can't help letting you know how much I appreciate the phonograph which came to me on the morning after the 8th, which was my 80th birthday. It was indeed a birthday gift from Jehovah, to be used in proclaiming his name. May grace and strength be given me to do with my might what my hands find to do." Watchtower 1940 Jan 1 p.16
In 1951, that the Watchtower stated the Witnesses were no longer to celebrate birthdays.
"Is it proper to have or attend celebrations of birthday anniversaries?-F. K., Nevada.
Such celebrations have their roots in pagan religions, and not Scriptural grounds. Some Bible commentators suggest that birthday celebrations may have had their origin in the "notion of the immortality of the soul"." Watchtower 1951 Oct 1 p.607 Questions from Readers
There is nothing unscriptural about celebrating birthdays. (See Birthdays for detailed information on why the Watchtower ruling against birthdays is wrong.) Most Witnesses struggle to explain why they can't without some embarrassment at the shallow reasoning used. Romans 14:5-6 advises not to judge others and their celebrations:
"One indeed judges a day above another; and another one judges every day alike, Let each one be fully assured in his own mind. The one minding the day, he minds it to the Lord. And the one not minding the day, he does not mind it to the Lord. The one eating, he eats to the Lord; for he give thanks to God. And the one not eating, he does not eat to the Lord, and gives thanks to God."
Once more we see that so called pagan doctrine and practices promoted in the Watchtower by the Governing Body were not replaced by new light until decades after Jesus is said to have cleansed his people. This is in sharp contrast with Christian times. How many times did the Apostles issue a decision under God's direction through the Bible letters, only to later decide that it was wrong and introduce an almost opposite viewpoint?