Birthdays in Indonesia mirror my concept of what the "holiday" should be. In Indonesia, the person whose birthday it is will give gifts (typically bring some sort of food or provide lunch for classmates/friends/etc) and this makes intrinsic sense to me. On your birthday, you did nothing aside from come out kicking and screaming, thus you should be working to repay this debt on the anniversary of that event, especially to your mother!
Furthermore, I have found Indonesians to be more forth-coming about their ages. Americans are often coy in regards to their numerical age, however, it was recently a co-worker's birthday, and she divulged her age to me without nearly as much concern. Granted, I was playing around in a similar fashion to my grandfather, and so it might be that Papa's strategy is exceeding effective in getting people to share their true age :)