The Red Devils legend has hailed the 71-year-old who recently added a 13th league title to his reign at Old Trafford, and has revealed he is unsure when the Scotsman will retire
United beat Aston Villa 3-0 on Monday to regain the Premier League title from Manchester City with four games remaining - making it 13 league title under Sir Alex during his 26-year reign at Old Trafford.
And Charlton has revealed the Scottish tactician never talks about calling time on his spell as manager.
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"But we all get older and Alex will get older, the decision will be made and it will be a sensational performance because he has a great talent for winning.
"He wants to win and I think some of the newspapers would love to find out what Alex is going to do with his future. But I can't help you, because I don't know.
"I don't ask any questions, either, just in case I trigger something!
"But he's been marvellous and everything that Manchester United wanted. He's been a major success and is the greatest manager there's ever been. It's unquestioned.
"But if you wanted to find out when he's going to change and have someone else in command, the only person who will ever give you a proper answer is Alex."
"He is such a motivator and he's in control totally of his squad. I'm happy just to say 'on you go, Al, you just do it the way you've always done it'.
"It shows in the performances and the goals that we're scoring. We have lots and lots of goalscorers now and most of them are pretty young, so the future looks pretty good.
"He reads the game and knows the game. He finds that young players are really important in the future of the club.
"Then when you've got someone like Robin van Persie, at 29, he has been a success. Alex wants the younger players who come through to be that successful too.
"How he does it is a question of talent that only Alex can answer. He knows everything about the game."