ren falling for haru isn't as far fetched as some seem to think.
ren was severely abused and treated like an animal before he came to live with haru's mom. haru brought him out of his isolation and taught him all his basics; i.e. how to hold his tableware, the need to wear shoes, and so on. haru was the first decent person to connect with him (haru's mom made things worse at first), and in this story ren is completely haru-centric. his world, especially at the beginning, revolves completely around this person. part of the ongoing story is about ren learning to make connections with other people.
as for people saying that no kid would act as mature as ren, again, i disagree.
ren is extremely intelligent, and most highly intelligent kids act older than their age. add to that his early horrible traumatic experiences and you've got a kid whose brain runs at light speed and whose life experiences far outweigh most kids his own age. hell, his life experiences outweigh a lot of adults i know now.
and on top of that, once haru "tamed" the wild beast, ren spent the next several years in the company of adults. only adults. its no surprise to me that he would be far more mature than kids his own age and i'm surprised, given this background, that people seem to think he acts too mature for his age.
on another note, the japanese school system is different than ours. ren would have been 15 when he graduated jr high, not 13. so he's 15-16 when they move into the house. whether you count that as shota or not is up to you i guess. most kids fool around at least a little by that age and i don't have a problem with it in this case.
i also don't really have a problem with the age gap. there are a TON of shoujo stories out there with a 15-16 yr old high school girl dating her teacher or someone else with the same age gap so it seems hypocritical to condemn it in this story just because they're both guys.
lastly, they're not blood-related nor were they raised together. there's no incest taboo to break because there is no incest.
as you might guess, i like the story. i always enjoy abe miyuki's work, which tends to make me stop and think a bit, and the art is lovely.
for those who find the contents offensive but enjoy the art i suggest you try Komatta Toki ni wa Hoshi ni Kike! instead, its barely shounen ai and the mc is an only child.