I can't find the chart but I remember seeing some demographic data from a rural part of India where, after a certain age, it started to look like stair steps with discontinuities every 5 years. Did they just have a huge (well-timed) baby boom twice a decade? The real story IIRC was that most of the elderly people didn't care to keep track of it and would just round their age to the nearest 5 years when asked.