My 2p - birthdays are usually childrens celebrations - the older we get , another passing year becomes less and less something to celebrate. The WTBTS has always been very much negative on children - most GB members and their wives (if they are married) are childless , as are most Bethel workers , so there wasn't really any problem in banning them. In practice , I know many JWs who go out for a big family meal on a childs or adults birthday or who have annual "present days" for their children (though not on the specific day). As one elder told me "there are ways of gettting around the Societies restrictions.
Bizarelly the society approve of parties for anniversaries and these have become major events in JW land. Notice the weird logic in this Watchtower article from 1977 .
w77 4/15p.253 OurTwenty-Year Project—Being Parents
"For instance, each year we have a big party on our wedding anniversary, with gifts for all four of us. We stack these on the piano days in advance, and enjoy a special meal together. It is not just our anniversary, but it was because of our wedding that the children came along—so it is their party too."