don`t know about that whole "mature" gamers thing...
I don't have time to research, but I'd bet on the whole there are a lot more M rated Sony games out there then Nintendo ones. If that is true, then Sony games target more 18+ gamers, correct?
Looking at Nintendo's big franchies like Mario, Wario, Zelda, and Pokemon, these are all G rated games. All of these games are also very cartoony, or very appropriate for young viewers.
Looking at Sony's main franchies like Metal Gear, Devil May Cry, GTA, etc., they are mostly M rated games, or at least T. The audience for a substantial amount of Sony games is 18+.
Of course, not all 18+ consumers are more mature then 10 year olds, but a fair amount are. A lot of these Sony games certainly target mature gamers, and deal with more mature themes.
Making any more sense why people tend to think of Nintendo's games as targetted towards kids?
I am not claiming that the Nintendo games are not fun, or that older gamers can not enjoy them. Some older gamers do enjoy them, which is fine. The games themselves though, just target and different audience and deal with simpler themes, or themes appropriate for children (which helps get the ratings that Mario, Pokemon, etc. do).
I do not mean to make fun of older people (18+) that like Nintendo games. Fine. A bunch of them are very fun. You don't have to get so defensive.
One example for me, I love Advance Wars. But when I play, I always think it is pretty odd that teens or very young people are in charge of armies and defending the country. How many US soldiers would follow a 19 year girl in a tank top? Themes and appearances like this are common for videogames though, but more common it seems for Nintendo products.
I had a hard time getting into Final Fantasy Tactics on the GBA simply because I was fighting with a 10 year old knight. Yes, the FF Tactics play system was very good, but the overall story and theme behind the game was very silly to me. The first Final Fantasy tactics game had an older protanganist, which fit better for me. Tactics Ogre had older characters as well, so does the excellent Fire Emblem.
Anyway, Nintendo games and play mechanics can be, and are good, but the themes and target of the games are targetted at children, as that is where Nintendo makes a lot of money.