Seasons 1-4 great for kids. Season 5 only for teens and older.
The 14+ rating for this show is well deserved for the fifth season, but it's pretty unfair to rank that for the entire show. I watched the show when I was about 7 years old and it was never too bad. Seasons 1-4 are generally family friendly; Sure, they're a bit darker than most other cartoons, but they're not so dark that it's like a "everything's hopeless, the world is brutal and cold" type of dark, more like Avatar the Last Airbender (which, ironically, the voice actors for Aku were also in).
But anyway, now that age rating is out of the way, let's talk about the show itself. It is a very creative and interesting show. For one, the plot completely deviates from other cartoons, let alone cartoon network shows, and brings a coherent plot rather than an episodic formula which makes it more interesting. I liked all the characters, many of them left an impact on me even if they were only in one episode, there's something to be said about a show that can do that. But speaking of characters, Jack was an amazing character, he wasn't the typical kind of hero who always prevails or always has a positive outlook on the world. He didn't have a negative outlook, but he seemed to understand better than other cartoon protagonists that the world isn't a perfect place (especially in this cartoon, but that's beside the point) and strove to make it a better place than it was. He never expects a reward for his actions because he has personal reasons for all of them, and helps anyone in need without even negotiating anything. To me, that made him a very interesting character, he didn't deviate too far from the heroic type and was a very interesting character nonetheless, kids can learn from that more effectively than other shows.
The tone of the series was very interesting as well. It wasn't an outright comedy and knew when to be comical and more importantly, when not to be. Jack goes through plenty of losses in addition to his successes, but he always perseveres through them and doesn't let any loss, big or small, stop him. The world also has plenty of bad people, but a great amount of good people too, and as I said before, all of them made an impact on me.
But anyway, I'd like to reiterate, DON'T SHOW YOUR KIDS THE FIFTH SEASON. It was put on Adult Swim for a reason, it's much more violent and dark than seasons 1-4. That aside though, seasons 1-4 are rated TV-Y7 and the violence is never graphic or anything (since Jack mostly fights robots instead of humanoid creatures), so don't fret over it. If I could give seperate age ratings for Seasons 1-4 and Season 5, I'd give the former a 7 year old rating and the latter a 13 year old rating. It's a great show for kids that's far more interesting and inventive than many other cartoons I've seen, even modern day ones.