Today "adult" often means "there's sex and/or gore", but the content is still simple and juvenile. But Grim Fandango isn't like that, it's just full of themes that probably confused the heck out of almost every kid that tried to play it.
Like the very first chapter throws you right into office politics. You deal with stealing a job from another salesman, sabotage a pneumatic tube messaging system, and sneak into your boss' office.
It all makes perfect sense for adults familiar with office work and all the movies it references. But I recall I tried it when I was maybe 14 and I couldn't make head nor tails of it. I didn't even realize the pneumatic tubes were actually a thing.
Things like Monkey Island and even Full Throttle are far more accessible.