Adoption was also used in western countries when gay marriage or similar systems (like the PACS in France) were not implemented.
Can you adopt a person who's above 18? I thought you can't adopt someone who's legally an adult. Is it different in japan? Well in my country you can't.
Yes, it is legal, it's the case in many countries. It's more for inheritance than anything usually, though the specifics and who must approve of it (if you already have children or you're married for exemple) varies by country.
Just to add to the T/N at the end, in Japan same sex couples do add each other in their registry, but as father and son, one adopts the other, which may sound weird, but sometimes you do what you gotta do to legally protect your partner even if it means bypassing the system