Well, I am gay and well before puberty or any pronounced sexual awakening, I was aware of some "gay" tendencies. Even more significant, I recall having "childhood crushes" (probably somewhere around the age of 7) and they were NOT for girls. So, how do you respond to that? There are many many other anecdotal stories from other gay people with similar stories (and some I know who are are "closeted" and have no interest or political agenda (as you suggest) for making such assertions.
Also, the picture you posted may not be real. On many social media sites there are pictures an GIFs with captions saying just about anything, so I wouldn't let something like that be the impetus for your "irritation."
Also, there was some discussion above...sexual orientation and gender identity are absolutely different but there is no need for me to post a wall of text regarding the differences when you can simply google it and research it for yourself, if it is of interest.
The topic is not what you know of being gay now. It is what you knew then. No one is saying you didn't have feelings then, but what you understood about the feelings you had at the time and age you had them. I don't think I can be more clear than that. If someone can be, please do so. Thank you.
As far as the picture goes, it may very well be fake, but the question still remains about the childhood recognition of degrees of attraction.
You must account for the historical context and social milieu in which a 7 year old is making such declaration. A gay youth thirty years ago vs. a gay youth in 2013 are likely going to produce two very different levels of understanding. Seven year olds in present day society are likely to be much more aware of what the term "gay" may mean. Obviously there are levels of attraction based on age and a prepubescent individual is going to lack the actual sexual portion, so there is your answer...they can only know they are gay to the degree to which they are developed and no one can give you an age as you originally ask. In addition, there are various psychological theories that likely require consideration such as Fredian theories which seem to assert sexual development extremely early with very slow and gradual development, but that is going to depend on what theories you buy into. Aside from the original question, would you apply the same logic to a similarly heterosexual framed situation? If a male child said, "I think (insert female name) is pretty and I want to marry her"? Would you stop and ponder, "now how can he be certain he is heterosexual?" You must ensure you're not applying heteronormative thinking. And, if you are wondering why a 7 year old may actually declare or use the term "gay," it is most likely because we live in a heteronormative society where heterosexuals don't have to "come out" or declare their orientation (so there is no attention given to it); however, the homosexual is a minority and is more likely to make declaration of their status as not to be mixed into or presumed to be something that he or she is not. And again, kids now are more aware of the term "gay," so naturally it is going to be applied. In conclusion and to relate back to what you just asked, "...but the question still remains about the childhood recognition of the degrees of attraction" well, it is just that...a 7 year old is going to understand the terms of being gay to the fullest degree he or she is aware of his or her attractions (this may just be simply feeling someone of the same sex is aesthetically appealing in a non sexual way) ranging all the way up to an 18 year old who is most likely fully sexually developed with a better understanding and actual sexual attraction to someone of the same sex.
From the perspective of a gay man, that's the best explanation I can offer. Hope it helps. :)