Everyone develops at different rates, so it's impossible to pin down a minimum. Here's my guess based on my experiences.
I got into the Pokemon game when I was 13, Christmas of 1998. That's probably not the truly minimum age, since I breezed through it in a day's time (would have been an actual day if not for a very embarassing mistake I made). I had played RPGs before, such as Dragon Warrior, so this was hardly new to me. I'd say you'd have to be, oh, about 7 or 8 before you can really start enjoying the game, and at 10 the game gets really easy.
Now, with the effort value thing, honestly, I think that's something you need someone to tell you about before you ever consider it. That's certainly how it was for me. I'm 20 and this is the first place I had ever heard about them. I still barely get it, because I know there are little intricacies all over. That could still just be me, so I'd say late teenage would be the minimum for understanding all the little things about the game. I still think you need to be told about it before ever grasping it, though. I knew my trained pokemon tended to be stronger than wild pokemon even when the wild ones had higher levels, but never figured out why until I came here.
Minimum to play: 7 or 8
Minimum to truly understand: 16+
In maximum, while everyone is entitled to what they like, I think someone past 35 who plays would be a very rare thing. Even if Pokemon still lives and continues to be the level it's at now (relative to other games), I don't see myself as being a Pokemon player at 35, much less 40. Obviously that wouldn't apply to people like Shigeru Miyamoto and (I guess) Michael Jackson, who take comfort in continuing a child-like happiness into adulthood. (Yes, I made a positive reference to Michael Jackson. Deal with it.)
