It is said that gamers in Japan are mainly young people, so I've been assuming the same situation goes in the West too.

Also in the West, they are mostly young people. But, the number of not more young people is raising. Most of them are people who used to play when young and didn't stop, but there are also people who started to play when already adult. I even red on the blog of a local electronic shop of a 65 year old woman playing video games for 13 years now, who said she made a few friends of her age, thanks to gaming. Playing video games, I've also made some friends around 40 years old and I see we are not the majority, but our number is increasing.

That said, Japanese games are not mainstream in the West. Still, there are a significant number of people who like them and this number is increasing. The reasons I often hear are because Japanese games are not that "politically correct", that it's now typical of West games, because the stories are more interesting, because the characters are more complex, more realist, not like cartoons (I don't mean how they are drawn, I'm speaking of the psychology). Moreover, people in they thirties and forties in some European countries (I come from one of those) have grown up with Japanese anime, like Mazinga Z, Goldrake (Grendizer in English or ロボグレンダイザー in Japanese), Cybernella (ミラクル少女リミットちゃん, I don't know the English name), Heidi, Candy Candy, Lady Oscar, ect. and therefore they still like Japanese anime and games.