"Around midnight, his family made him cut a birthday cake. It's also his son Ameen's birthday which makes the day even more special for Rahman," a source close to the Mozart of Madras told IANS.
The source added that a party has been planned by Rahman for industry friends on Friday evening.
Filmmaker Bharat Bala was one of the few guests to personally extend birthday wishes to Rahman.
His fans on social media have been trending the hashtag ARR 50 since Thursday night.
On the special occasion, Rahman's sister Isshrath Quadhre has dedicated her Mash Up cover version of Rahman's songs to her brother.
The video will be played on several television channels on Friday.