The article on birthday celebrations by jwfacts was nicely written.
Another detail to add would be that birthdays would had to have been celebrated before and after the flood. A number of people mentioned in Genesis knew exactly when a person was born and died in the listing of the genealogies. That celebration would be the reminder of how old you were in the stream of time. We knew birthday details of Noah, Shem, Abraham and many more faithful people in early bible times because it mentions exactly when they died.
If God thought birthdays were evil he would have spelled it out the prohibition in the Mosaic Law. Since he did not do that then celebrating birthdays is not a sin and the jw leadership is presumptuous on God's thinking. One of the first kings of Israel was accused of presumptuousness by God and we knew what happened to him. God left him and would no longer give him divine instructions. Same thing has happened to the leadership in the jw religion.