Cumbria-born keeper who joined the Reds as a 14-year-old from Carlisle United.
Henderson spent six years with Carlisle after initially starting as an outfield player and he reportedly attracted attention from Chelsea and Newcastle with his performances.
An England youth international, he is rated as an outstanding prospect and was tracked by United since the tender age of 10.
"Dean has the potential become an excellent goalkeeper," commented Academy goalkeeping coach Alan Fettis. "He has that competitiveness about him and doesn’t want to be beaten at anything. It’s a very healthy attribute to have, of course.
"However, his major asset is that he is technically good. He’s still a young keeper who works hard to improve, which is what is expected to play for Manchester United. He has got potential but it’s up to Dean to keep striving to improve and, if he does, he could have a good career, as long as he listens to the advice given to him here and works hard. His attitude towards training and games is always first class."