As the discussion above shows, this is a complex topic. I'll add a few of my own thoughts here.
As for underage material, as @bi4smooth points out, you can't simply look at a "model" and determine their age. While a pre-pubescent 10 year old would not be mistaken for an adult, someone who's 15 can look 20, and vice versa. It would be nice to be able to rely on a producer's certification of age-compliance, but this is not infallible. Some get it wrong, and some purposely misrepresent the facts.
Then, there's the issue of amateur material--anybody with a smartphone can make and post a video. There's no way to determine the age of the performers, and thus some sites will not host (or share) material that isn't from a commercial producer. There is amateur material on GR.ru, and it's possible that there are some underage performers. The moderators can't look at everything that's uploaded.
Part of amateur material is voyeur content--filmed in a locker room, shower, or over the divider of a toilet stall without the consent of the subject. These subjects could pursue both the person taking the photos and anyone possessing that material for an invasion of privacy--the usual norm being that photos or videos taken where a person has a "legitimate expectation of privacy" constitute such invasion. This gets further complicated given that there's no way to verify the age of a person being recorded in such circumstances. I have seen some toilet stall videos where I am less than convinced that the subject was 18 or over.
And finally, we need to distinguish age of consent from age of majority: In Canada, and most of the United States, it is legal to engage in sexual activity with someone 16 or 17 years old--but possessing a photo of that person's genitalia would be a crime.
One conclusion from all this is that each of us needs to use caution about what material we download and/or keep on our computer. Ultimately, it doesn't matter that you believe it's "legal" or can make an argument for that being the case, but rather it's what the legal system would think if you come under scrutiny. There have been cases where a person's computer or download records were seized for one reason or another, and when an investigator had reason to believe that there was banned material present, criminal charges were made. These might be later dismissed or acquitted, but only after considerable inconvenience and/or embarrassment. I don't say this to frighten people, but only to recommend that all of us be careful in what we do. Unless one is really into child (i.e., pre-pubescent) material, there's an abundance of clearly legal content available on this site that will be more than satisfying.