I'll get the basic facts out of the way first. Warframe is a Third Person Sci-Fi Shooter with a fun melee combat element and an interesting class system. For those of you averse to guns, it is entirely possible to exclusively use melee weapons. Bows, throwing knives, shuriken, and energy weapons (although all but one laser must be acquired via Clan Dojo research, more on that later) are additional alternatives to the usual sci-fi firearm.
Monetization: This is an issue here, as the only easy way to get a weapon or usable Warframe class is through buying them in the store with Platinum, the game's cash shop currency. This means you will be dishing out money to get these items in a timely fashion. Crafting is an option, but requires rare drops (which may also be bought with Platinum) and will take anywhere between a few moments or a full 48 hour period (which can be bypassed with, of course, more Platinum).
Violence: Here's the thing: Common Sense has written a horribly inaccurate review. It's clear they didn't play past the 45-second tutorial in which the only violence is against training holograms. This is a major no-no in the field of review writing. It is actually so bizarrely wrong that I half-suspect money changed hands between the review's author and the game's developers. Although a toggle exists in the Options menu to disable gore, it is on by default, and is not subtle about it by any stretch of the word. Most melee weapons and some ranged ones will dismember their opponents, cleaving off limbs and even chopping their opponent in half at the waist, diagonally at the shoulder, or straight down the middle. As these are humanoid opponents and melee weapons do not cauterize, gore is clearly visible. Raw-meat like textures and bones can be seen no matter what happens, from a part of a spine, to half a rib-cage, to even the inside of the spinal cord and skull, through which half of the opponents brain is visible (when chopped apart at the waist, on an angle, or from head to to, respectively). Damage causes characters to bleed profusely, staining the environment. Again, the gore can be toggled off, but I believe the blood remains (although possibly toggled down). With gore off, the review is far more accurate, as it becomes a harmless sci-fi action game with some creepy environments. The Boltor rifle and Bolto/Akbolto pistols fire glowing darts that impale enemies to walls. Orokin Derelict missions, only accessible by crafting keys to access them with, are overgrown space-palaces, dark except for your character's helmet-mounted flashlight's glow. This gives it a feel similar to the Dead Space series. The Infested are a vaguely disturbing enemy, and some have an affect that drains your character's Energy Pool while the head's up display dissolves into static for a few moments.
Language: One instance of profanity in an item description for the Dethcube, which describes it as a "bad-ss cube of 'Deth'".
Sexual Content: "Female" Warframes have curvaceous figures (especially Saryn), but nothing more than that.
In conclusion, with gore off, its fine for all ages. But, as the gore is still relatively cartoonish, if your child can handle it, fine. Play it with gore on for a while as a "pre-screening" and make your own opinion. I know I handle it perfectly well, but that's just me.