Perfectly legal, provided all the correct procedures are followed as would be the case with an infant. I assume that the adult to whom you refer is a person aged 18 years or over ?
The Adoption Act very clearly states that to be adopted a child must be under 18 (and never have been married) and there must be consent to the adoption order from both parents unless such consent can be dispensed with.
What do you hope to achieve? There may be other options.
I would be inclined to accept the answer provided by Ethel* (See immediately above), since posts provided from that source are normally very carefully researched, so I am prepared to withdraw in part my answer provided earlier.
But...have you considered the Human Rights legislation ?
I'm sure Ethel is right - I found the Act which says 'must not have attained the age of 19'. Anyway 'adoption' is about assuming 'parental responsibilities' in the Act. It's not the same as metaphorically 'adopting' someone. I have my answer now thank you all!
My Partner's daughter would like to be officially adopted by him. She is nearly 18 years old so it's too late for a child adoption. How do you go about an adult adoption?