How do I adopt my partners 21 yr old daughter

Is she disabled? Why does she need adopting? Adults don't adopt adults.

At 21 years of age I don't understand why adoption would be necessary, unless, like Wondergirl said, she's disabled and needs long term parenting. But generally adopting an adult isn't something you do. Adoption is normally done to become the legal parent of a child. It's done so that you can care for the child, enroll the child in school, take the child to the doctor, house the child, and other things that only a parent can do. An adult doesn't need that kind of care, therefore adopting an adult is not done, unless the adult cannot care for herself because of a disability.

Another thing. You said that this is your partners stepdaughter. Did your partner legally adopt her? Are you and your partner married?

https://en.wikipedia.org/wiki/Adult_adoption

Adoption is forever. Not a good idea to adopt a 21 year old.

Typically, most states set the adoption age at 18. However, it depends on the circumstances. Delaware stated that it was okay to use adoption between adults to create inheritance rights if that was the primary purpose of the adoption.

What is your reason?

I see that the OP changed their original post. Originally he said that this was his partners step daughter, which led to me questioning if the partner had adopted this 21 year old.

This is just a big mess. If the partner isn't the legal parent of the 21 year old, then the age of this person won't matter, there's no way the OP would be able to adopt a step child of his partner if the partner himself isn't a legal parent. Then add on the matter of the age of this "child", and it's a whole other bag of worms.

The title is where step-daughter is mentioned. I think it was essentially an editing issue. He probably started to say his step-daughter and changed to his partner's daughter. Adding one word and forgetting to make necessary changes happens a lot.

Mark, without knowing your location, we can't even tell you if it is possible for you to adopt her.

Is her other biological parent alive, in her life, left her, or something else? Have you been acting as her parent for a majority of her life but unable to adopt her before now for reasons beyond your and your partner's control?

I understand that there is probably a bond between you that you both would like to make stronger/legal and/or show the world that you are "family". Adoption may or may not be the solution to the reasons you are looking into it. As has been said by the others and Joy's link, Adult Adoption sometimes come with restrictions that might keep it from doing what you want it to do.

If she considers you one of her parents and you consider her your daughter, then you might look into wills and other legal means to support that relationship.

If you and the other people involved love each other and consider yourself a family, that won't change just because you can't make it a legal arrangement.