One thing you MUST know BEFORE you allow your child to see it is whether or not it is the uncensored or censored version of the show. The best way of telling is simple: the uncensored version begins with a shot of a mountain, the censored starts with something (a pod) flying through space. It may also say somewhere (the 4Kids animation and Nickelodeon are the censored ones).
This is critical when deciding. I will discuss both.
The uncensored version first. As a point of reference, if you would allow your child to watch Michael Bay's Transformers, you would allow them to watch this. It includes multiple reference to sex, and one character in particular swears on many occasions. It never gets worse than "D*mn it" or "That b*stard", and a few middle fingers are shown.
The sex references are a bit stronger. Non-sexual full frontal male nudity is not uncommon, but it's not focused upon, either. It's just sort of... There. They treat it naturally. Female characters will frequently use sex appeal to their advantage (OK, not that frequently, but it's a regular occurrence). Many males are portrayed as perverts.
The violence should only be a concern if you're watching the censored version (in which ALL of the above are void- except some mild flirting from females). There isn't much I, or anyone else, can say on the subject. The entire show is based on martial arts fighting- that alone should be a deciding factor. It's not tame, but it's also not brutal. Note the censored version, as you'd expect, omits most of the blood.
If I had to give an age rating for the censored version, I would say:
ON for kids age 8