Just thought I'd give my two cents. I am a 25 year old man.
I grew up playing GTA 3, GTA Vice City, GTA San Andreas, GTA 4 and now GTA 5. I got GTA 3 in year 6, age 10.
When your kids are telling you that all their friends have it they're not lying to you. All of my friends had it. Yes they're 18 rates games but it depends on the individual and their emotional maturity which should determine whether they should be allowed to play it or not imo.
The game, like real life, involves murder, drugs, prostitution etc. The latter being a very small optional part of the game. In free roam it is as violent or calm as you want it to be. You're in control. As a kid me and my friends found it funny to live like a normal person. Driving legally and stopping at traffic lights, going for food, getting a hair cut. All sorts.
In GTA 5 you can play golf and tennis. Play darts. Race with friends. You can even be a taxi driver. There are countless things to do that do not involve violence in any way shape or form.
I am due to have my first child later this year. I would have no issue with him playing this type of game when he is older assuming I felt he was mature enough to handle it.
A big argument for kids not playing GTA is that they will replicate what they see in game in real life. Myself and everyone I know who have played the GTA series while growing up are all now well adjusted adults with families and good jobs. I can confirm I have never had sex with a prostitute and then proceeded to run her over with my car after paying her. Shocking, I know.