Age Difference
Okie, first we must note the fact that Ryuichi is 31, and Tatsuha is 16. Yes, that is a fifteen-year age difference. Here are some important age platforms to consider...
→ Age of consent in Japan is 13
→ However, all municipalities and prefectures have their own laws regarding this.
→ → Tokyo's "Youth Protection Law" prohibits adults from engaging in sexual activities with anyone under 17
→ However, 20 is the age when someone is considered an adult
So, by law, Tatsuha is legally entitled to be able to say 'yes' to a relationship with Ryuichi, but at the same time their relationship - if sexual (which considering them, it would be) - in Tokyo, at least, would be violating the Youth Protection Law.
That puts our boys in a bit of a pickle, doesn't it? But knowing them, they wouldn't care too much. However, as it has been noted in many fics and discussions, being in an illegal relationship, especially with another male, could seriously affect Ryuichi's career in a number of bad ways. It is up to personal debate on whether Ryuichi would worry about that or not, I, however, tend to subscribe to the fact that he would.
Career Problems
Being the international superstar that he is, the spotlight and media is constantly on Ryuichi. His careers seem to be his passions in life, and while he has more than one, they are important enough for him to travel to wherever he need be to fulfill them.
However, with the varying and busy schedule of rock stardom, having much of a personal life beyond that would be difficult. Sure, he has time to play around and color with Shuichi, but his job takes up a big time of his life. That'd be a problem if he was in a relationship with Tatsuha. I've always seen Tatsuha as someone who wants/needs reassurance of someone's affection, but that's another essay :D
So, what could they do about this little career issue? I think Ryuichi'd try to keep it a secret. It might also add a little spice, having to hide it from the world, you know~ While I think that Tatsuha would want to be more open about it, but he would go along with keeping it a secret for a few years, at least - anything for Ryuichi!
Family Problems
Tatsuha has a lot of responsibilty with his family. He is the new temple-bound man, and not only that, since is he is 16, he is still very much under the influence of his father. Hey, let's admit it, it's always different living under your parents' roof, ne?
While Tatsuha doesn't seem to be the type to adhere to some traditions, requests, and, er, laws, he is nevertheless affected by his immediate family, and in particular his father. Not only would his father be disapproving - clearly seen with Eiri's relationship with Shuichi - but I'd think he'd probably call the authorities, considering Ryu is 31. Then he'd ground Tatsuha for eternity.
At least that's what I think u-u;
Silver Lining
Gesh, after all these issues you'd think this is an Anti-TaRyu site, ne? u-u But far from it! What I think is most important, is that despite these differences, I think they can find what they are truly looking for in each other - some sort of stability, friendship, trust, and of course, love and good sex.
While Ryuichi is quite a bit older, he also can be rather immature quite often, and childish, as we know. Tatsuha doesn't seem put off by that, and just seems to think it's cute! He likes both sides of Ryuichi, the child and the adult. Their respective ages seem to balance out, because while Ryu is an adult, he is not at the same time. Tatsuha is a child, yet very not as well, but he still has his moments.
Despite the age difference and the problems that come with the package, Tatsuha and Ryuichi, in my opinion, would be able to suck it up and carry on for the ultimate goal - a happy, working, supportive, and loving relationship. They both are rather good at making their current situations work well for them, so why can't they with this too?
Love knows no age!
