A statement from the club read:
“The player passed a medical in Manchester this week and has agreed a five-year contract.”
Although no fee was mentioned Sky Sports reported a fee in the region of £16m.
Young, speaking in an exclusive interview with MUTV said:
“It’s a chance to… hopefully become part of the history by helping to win the 20th title.
“It’s a prospect I always looked at from being a child: to play for one of the biggest clubs in the world.”