It could be seniors, adults, young adults, teens, or kids but IDK!!!
I would say it depends who is playing and at what age as their is no definite best age to play rather I would say that the best age to play chess is completely irrelevant and chess is a rare thing in today's world that anybody of any-age can do!
Seniors. Because we take the time to actually think about our moves and are not hyper fixated on our rating. We are interested in the history of chess and don't see it as some "leveling up" type of game.
Normal chess doesn't require strong muscles or fast reflexes, but does depend on study, logic, memory. The brain may lose some strength of functionality, such as speed of learning and retention, but it can be more than made up for with experience, studying, and practice. All in all it equalizes. Only in the latest years of one's life does the brain suffer to the point that chess skill rapidly declines.
So, 12 - 70+ years at the extremes.
I think 12 - 25 year old people. It's when GMs are at the peak of their chess career.
I now think everyone can be good at chess if they try their best!
except for babies! 
At 30-40 players are strongest
https://ratings.fide.com/profile/5000017/chart
It could be seniors, adults, young adults, teens, or kids but IDK!!!