Most to all adults are initiates of a cult, regardless of culture, and around 2% of initiates are Rune levels. Many people are lay members of one or more other cults.
These are not hard numbers of course. Thanks to war, pressure, desperation, or whatever, with some cults the number might be as much as 3% and in others it might be closer to 1%.
For example we’ve got about 1.2 million Lunar initiates (all cults) in the Lunar Heartlands (total population 5.5 million), which gives us somewhere around 20-24,000 Rune levels.
While in Sartar we have 34,000 initiates of Orlanth, but might have somewhere around 1200 rune levels thanks to the pressure of the last few years.
Just about every Gloranthan culture defines adulthood as initiation into a cult. That often takes place over several years and is usually somewhere between 15 and 21.
This is true for just about every human culture – Orlanthi, Pelorian, Lunar, Malkioni, Praxian, etc.
As a general rule about 66% of any listed Gloranthan population are adults and about 33% are children. Compare that to the modern US where only about 18% are under 15. I am really not interested in going more granular than that.
As another aside, I figure most initiates only make the single POW sacrifice and don’t continue going much deeper into the cult mysteries. They are likely concerned about other things like work, relaxation, sex, family, more work, avoiding work, feeding the chickens, etc.
Does this apply to the Elder Races in addition to the human cultures? No, only humans. The Elder Races are all over the map. The dwarves and dragonewts are entirely other, the trolls are bifurcated species, and the elves are plants. Don’t assume they work like humans do.