The 21-year-old has signed a four-and-a-half-year deal, believed to be worth £30,000 a week, with Villa, and could cost up to £9.65 million for the club.
Aston Villa manager Martin O’Neill said: “He has got enormous potential and I think he will fulfil that.
“Ashley is a really talented player. I have seen a lot of him and I am going to back my judgement.”
Young could make his debut for Villa against Newcastle on 31 January.
O’Neill added: “I had been hopeful Ashley would come to Aston Villa after he verbally agreed to join us over the weekend but you never take anything for granted until they sign on the dotted line.
“You think along the lines of `Will he be of value over the next four or five years’ and you base your decision on that.
“He can play on both flanks and he can play alongside the centre-forward.
“I think he is an exciting player and has got a great touch, great control, plus a wee bit of pace. I am excited with his signing for us.
“He is only a young fellow and it will be hard to put him under the sort of pressure where he is having to justifying himself from minute one, but overall I have no doubt at all he is good enough.
“I know his potential but it is different knowing the potential of a player and what they eventually go on to achieve. I believe he has the ability to go on and achieve things.”