Wait, isn't there that whole thing with e.g. family business owners adopting sometimes even adult guys to make them heirs, if one is lacking own? I remember hearing something about that custom.
You're talking about formal adoption of adults (usually by married but childless mature people) so they can continue the "family" business as formal family members (like a 70% or even 90%). I'm talking about child adoption and surrogacy by single people (Until the 80s, adoption without blood ties was generally prohibited). Many countries prohibit single people from doing this due to concerns that they will not have the capacity to raise and provide for a child, or due to concerns that the adult is pursuing "alternative relationship types." For example, just 15-20 years ago, Japanese officials proposed banning surrogacy altogether. It's also important to keep in mind that in Japan, you need permission from the child's relatives to adopt.
So, while it's not outright prohibited by laws, unless you have wholesome good family and are blood related to the child, you risk getting into bureaucratic hell.