I played this game for the first time in 5th grade. Was it appropriate for me? No. Was it a blast? Yes. I was a super-crime fighting badass and that's what makes this game so cool to me.
If your kid's killing innocents and selling evidence it's entirely their choice. It's tempting at times to go on a rampage, but it is indeed a healthy release.
Bad guys are bad, and the "good" guys who're bad gets what was coming to 'em.
There's sex shops and strip clubs, the pedestrians swear almost infallibly, and there's tons of gore, (beheading old women is a particularly gratuitous example) you can involve yourself in illegal fighting circuits and street racing syndicates, and the story from the start is about a drug cartel, but as the M rating suggests, anyone Seventeen or older can handle this.
(I'm fifteen and re-own it after trading it for Golden Sun on GBA)