My Mother started partaking around 1982 (she was in her mid-thirties at the time). She married a brother and soon after he was partaking. Soon, other people in the hall got "anointed-fever". I have not talked with them in years but I recently found out through the grapevine that my nephew and his Dad may be partaking in the next memorial. I'm starting to think there are high-levels of "midi-chlorians" in my family. ;-)
I have been out for 25 years but I am sure that the numbers are waaaaay higher than what they report. As already pointed out by posters, many partake in private and/or are "discounted" as "mentally ill/imbalanced", etc. I was an MS at the time and I remember given directions by the elders on who to count or not. Crazy.
Glad to be now be an atheist where I don't have to pretend to understand all of this nonsense.