I’m a 28 year old man can I be adopted by a 21 year old woman born in the exact year of 2000 to be my mother?

No.  I believe every legal code specifies a minimum age difference allowable between adopter and adoptee.  What you describe does not qualify.

Not possible.

In the case of a handicapped individual who cannot care for him or herself, a younger sibling or relative might be appointed as guardian, but not parent.

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You're too old to be adopted by anyone.  

I can't think of any situation where that kind of adoption would be a good idea, but to answer your question...

Adult adoption is legal in many US states, but the exact laws and restrictions vary from state to state. You should call an attorney in your own state to ask whether it's legal. Most attorneys will answer a simple question for you free of charge over the phone.

NOT FOR SEXUAL REASONS

Adult adoption is statutory. That means ya gotta read the statute and see what it says. First, figure out where you live. The relevant statute is in the adoption part of your state code. I know it is tragic to have to read and think for yourself. You have my sympathies.

Maybe.  But I don't know what benefit you would get from being adopted as an adult.

No. A mother can NEVER be younger than her child.