ok but they should’ve thought more about their age difference or how they would fall in ...

emotional attachment. taeheon was being abused his entire childhood and thats why he flinched at every kind gesture towards him. he was emotionally attached to sejoom bcuz he was the only one being nice to him. pretty sure he was just wanted to be protective of him at first, then developed into love next

yeah, i do understand the logic behind it if i stop to reflect, but honestly speaking i think the author just forgot to consider their age difference in childhood when planning the characters. despite it being possible to create a logical reason for his feelings by considering taeheon's neglected childhood, as you mentioned, it doesn't change the fact that they fell in love, since there’s even a scene of them kissing. not to mention that present-day taeheon himself drew a connection between sejoon’s present behavior/characteristics and his past behavior/characteristics, pointing out things like “his warm flesh, hot to the touch” (while they have sex btw) and other things that suggest the presence of a love/desire relationship and attraction rather than just an immature emotional bond between two children…

wait so he romantically liked him when they were kids?? help i just thought he got super attached and when they grew up he romantically like him then…

I'm not sure if he romantically liked him when they were kids, it was more like an emotional bond between them, finally being oart of a family.
Mild spoiler
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If I remember correctly it was Sejoon who kissed Taehoon when they were kids. It's been long since I have read the raws