And at 16 years and 10 months, he beat the previous SPL record held by David Goodwillie by more than a month.
Dons boss McGhee said: "That's fantastic. What a good goal; what a finish.
"He had one earlier in the season when he might have gone through and scored and he didn't score.
"When I saw that, I was just hoping he would go one better, so fantastic.
"That's why he's in the team, because he's got all these ingredients.
"You see him sometimes holding off men and then of course his running and now his finishing."
Aberdeen have been trying for some time to extend Fyvie's contract.
"All we're trying to do is prolong it and give him more money," McGhee added.
"At the moment, he's playing for almost youth-team money.
"We'd like to obviously fix that."
McGhee believes Wednesday night's display, which was capped by second-half goals from Darren Mackie and Derek Young, was the Dons' best of the season.
"I think it's the most complete performance," said McGhee, whose side moved up to sixth, a point behind tonight's opponents.
"We've had moments in games but I think it's the most emphatic performance.
"Hopefully, we can maintain that now - that's the challenge."
McGhee shrugged off a frank exchange with opposite number Csaba Laszlo during the first half, saying: "Touchline antics are part of the game and that's something I enjoy tremendously."
Wednesday saw Aberdeen cancel Tommy Wright's contract after the forward failed to establish himself at Pittodrie.
McGhee said: "Tommy wanted to get south again I think.
"Up here, he's suffered a lot of injuries."
The night was made even worse for Laszlo with the loss of Lee Wallace (groin) and Ian Black (back) to injury during the match, while Scott Robinson and Ruben Palazuelos suffered knee knocks.
"Some days in football you have everything come in a negative way," Laszlo said. "You lose a very important game which for us was not a game you should lose."
The Hearts boss believes some of his players were the victims of rough treatment.
"If you see Scott Robinson's knees, they are warm with blood; this is not an accident," he said.
"Ruben, he also had a kick against his knee. He must have a scan.
"Lee Wallace, he had an old, old injury. Now Ian Black with his back.
"To lose in one game three goals at home and to lose four players...
"We don't have a big squad; we have just young guys."
Hinting at his continual frustration at not being given the green light to sign players this month, Laszlo added: "The other young guys don't have the ability to go through the season at this high speed."
Meanwhile, the Hearts boss rubbished reports Celtic had made an official approach for left-back Wallace, saying: "We don't have any official request from any club for our players."
Source: Team Talk
Source: Team Talk