this is not a spoiler , just my theories :
theory 1 :
i guess in this manga world , there is this kind of technology : they can inject the DNA / body juice of subject A , a dead person , into subject B , a living person , and B 's face will be transformed to look like A , and B will gain the strengths , powers of A . but B can still be looking younger than A , and B can still be bigger , taller or smaller than A .
theory 2 :
- ohma : around 30 years old .
- ryuki : around 20 years old .
- old masaki : around 30 years old .
- young masaki : 21 years old .
so i guess there is a cloning facility that has operated since 40+ years ago . the customers will visit the facility and order : i want a strong son , made me a boy with strong genes and i will adopt him .
ohma and ryuki come from the same DNA , old and young masaki come from the same DNA . and all 4 of them have no real parents , they are actually adopted and raised by some people , because they are man-made clones .
someone ordered ohma to be made , and then a few years later , they dumped him in "the inside" .
the old masaki 's judo dad was actually his adoptive father .
there maybe more clones , with the same faces but different ages , walking around in japan , if the facility made them years ago .