Considering that the demographic that caught on during the third and fourth generations have aged a bit since Diamond and Pearl, I wouldn't put it past GameFreak to attempt to make the characters older. Besides, most people who play video games at a young age come face to face with older teenagers, young adults, and in many games nowadays, main characters in their thirties or forties. Pokemon was lagging behind with the two ten-year-old characters, but I'd say that they can only go a little higher age-wise for a main game. We saw people in their late teens or early twenties in the Ranger games, but I wouldn't want to start an adventure in my twenties.
we've seen the franchise lean towards more mature themes such as Religion, Abuse, Death, etc.
Where does religion become a big part of the game? Sure there's a so-called "church" and many people throughout the game pass down legends. The legends are there to develop the game and give it substance; we don't see people kneeling down and worshiping devoutly or identifying a text as a "holy book." Believing in legends does not mean that you automatically put those legends at religious status.
Abuse from the villains has been around for years, but it's always been considered a negative thing. What the villains try to do with the Pokemon would be considered abuse if we did it to animals in real life, and it has always been frowned upon and outlawed in the games. The games are not getting "darker" because the villains abuse Pokemon; we've seen this in the past. We also have to stop the villains from doing this; the games
would be getting darker if we abused the Pokemon ourselves.
And as for death? I personally think that death was talked about less in the fourth generation than other generations. Especially when you consider how much death was talked about around Lavender Town and Mt. Pyre.