When Ragnarok Online was a new game, and in the peak of its attractiveness, the years were around 2003-2006 (14-17 years ago).
Most people only play older, outdated games for the sake of their nostalgia, and nostalgia mostly comes from remembering experiences from your childhood or teenage years.
Children and teens who are the right age demographic for a MMORPG are around 9-19 years old.
Ages 9-19 added to 14-17 years of time equals the current age of people still interested in playing RO to be 23-36 years old.
People in their 20s typically still play video games a lot, but people over 30 typically begin either losing interest or realizing that they need to use their time more wisely, raising a family, or working a job, and don't play as often or at all.
So the way I see it is the average RO playerbase is around ages 23-30.