Adopted adult foreigner by Japanese national?

I have been living with Japanese citizen in Bangkok for 2 years. Now, she wants to go back to Japan but want me together, but because I am foreigner, I can not stay in Japan for long. She suggests to adopt me, can an adopted child of Japanese live in Japan?　She is 50 years old single.

I checked the Internet, but we are not sure.
"日本人の配偶者等　Spouse or Child of Japanese National"
I am already 23, some website says only 6 years under can live in Japan if adopted, is it true? Is there no other way for us to live together in Japan?

You can't be adopted as an adult.

Look into a different visa. You could marry her for a spouse visa. You could get a work visa. You could apply for a language school and get a student visa.

You can be adopted as an adult (it's fairly common in Japan), but that doesn't necessarily qualify you for a visa. Contact your local immigration authority to be sure, but it seems that you'd still have to qualify for a visa on your own merits.

Yes I know adult adoption is common for Japanese, but I am not Japanese citizen.

She considers me her own child so that's why she wants to adopt me and to live together in Japan, and since I'm still quite young, to work and provide for her, I can speak Japanese.

The problem is, some websites I check regarding Spouse and child of Japanese national visa has a rule which state that only adopted child under 6 receive that visa, which mean I won't get it since I'll be adopted at 23.

I don't know whether it's true or not, but would like help.

That's what I mean about you being able to be adopted, but that still not qualifying you for a visa. Your age disqualifies you from a child visa, so you'll have to look into other possibilies (which likely mean you'll need to qualify on your own merits). All in all, becoming adopted will likely only give you a slight advantage in moving to Japan long term. You should talk to an immigration expert to figure out your options.