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While it seems to be a combination of implausible and impossible, this Saturday sees Gianluigi Donnarumma celebrate his 18th birthday. Take a moment to consider that statement. A player who routinely stands between the posts for one of European football's most recognisable and iconic clubs is only now about to finally and legally become an adult.

He has already become the second-youngest goalkeeper to ever play in Serie A, making his Rossoneri debut at the age of 16 years and 242 days. It was Sinisa Mihajlovic who took the decision to field him against Sassuolo in October 2015, the coach tired of seeing error-filled displays from former Real Madrid man Diego Lopez.

What has happened since then has been the story of a genuine phenomenon. Donnarumma has made 59 competitive appearances for Milan, already recording 18 clean sheets in the league and earning a regular place in the Italy international setup.

Indeed, in August last year—aged 17 years and 189 days—he would become the youngest goalkeeper ever to appear for Italy, continuing the notion that he is the heir apparent to namesake Gigi Buffon when the Juventus skipper eventually calls time on his own illustrious career.

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His exploits over the past 18 months have been covered extensively, but what brought Donnarumma to that point and what made him such a stand-out performer at such a young age?

The journey to his current status as arguably Europe's brightest goalkeeping prospect began with his birthplace of Castellammare di Stabia. A small coastal town just outside Naples, it is a footballing hotbed, home to former Italy international striker Fabio Quagliarella but also rapidly gaining a reputation for producing talent that thrives at the opposite end of the field.

Bologna goalkeeper Antonio Mirante, the recently retired Gennaro Iezzo and Donnarumma's own brother Antonio—who also spent time with Milan—have all pulled their gloves on in the Italian top flight.

However, it was another member of his family who first helped out the young Gianluigi and set him on the way to stardom, as he explained in an interview with the official website of the Italian FA.

"My uncle took me onto the Castellammare pitches, holding my hand, and gave me the passion for this sport," he said. "I was only four but he could see my talent already. I started playing football in the team's academy as a goalkeeper and I never left this position."

From there he would rapidly progress, literally standing head and shoulders above his opponents. As an 11-year-old competing in those local leagues, he was often a foot taller than any other player, with his mother Marinella routinely required to produce her son's identity card to prove he wasn't too old for the category in which he was competing.

It was also during this time that he met Ernesto Ferrara, a coach whose reputation for developing young talent is renowned across the peninsula, and his latest pupil would attract nationwide attention.

"Thanks to him, I improved a lot," Donnarumma told the Vivo Azzurro website, "not just my skills as a goalkeeper though and from that, some Serie A teams started to look at me two years ago."

Inter Milan would soon look to sign him, but despite an impressive trial with them, the 13-year-old instead opted to follow a different path. "I chose to come to AC Milan and not Inter partly because my brother played here and partly because I have always been a Milan fan," Donnarumma said back in 2015 (h/t BBC Sport).

He quickly looked to show that was the right decision, and over the next two years he would astound even those accustomed to witnessing greatness. "A youth system produces one goalkeeper in a million like Donnarumma," former Milan defender Filippo Galli—who won five Serie A titles alongside Franco Baresi and Paolo Maldini and is now head of the Rossoneri youth system—told the UEFA website in November 2015.

As his impressive performances continued, the club's then-coach Filippo Inzaghi rewarded him with a place on the subs' bench for the final two games of 2014/15 before Mihajlovic replaced the former Italy striker.

But none of this was by accident, as journalist Augusto De Bartolo explained to Sky Sports during a recent interview about the progress made by the teenage sensation and his career to date:

"The achievements and success Donnarumma has experienced was all planned by Milan. When the club decided to give an opportunity to the keeper in its own youth team, the youth manager, Filippo Galli, was sure that Donnarumma would become a professional player.

"All of coaches in the academy knew that there was a plan around the growth of Donnarumma, but in truth, his first Serie A cap came even sooner than the club thought. He is a boy with his feet on the ground, focused on his career. He is impressive because he acts like a seasoned player."

As the 2015/16 campaign approached, a penalty save from Toni Kroos during an International Champions Cup tie with Real Madrid quickly went viral and kept his name in the news as he signed with agent Mino Raiola.

"I still cannot believe it," Donnarumma told the Italian FA website shortly after his debut at San Siro. "How to explain the emotions you feel in that stadium! Every time it's the same, when I come out of the players' tunnel and to go to the bench I relive my childhood dreams."

Now standing 6'5" (1.96m) tall, the boy who caught the eye of so many people within the Rossoneri ranks is about to become a man, with current Milan boss Vincenzo Montella in no doubt about how good he can become.

"For me, Gigio has the potential to win the Ballon d'Or," Montella told Milan Channel (h/t Goal). "A lot will depend on which team he chooses, he will have a bigger chance to win it at a big international club like Milan. It will depend on which club he plays for and his performances for the national team."

Those comments stem from speculation that he could be set to move on, largely thanks to the seemingly constant stream of comments from Raiola, whose most recent interview with La Gazzetta dello Sport followed an all-too-familiar pattern.

"His future? I think it's only right I said that that clubs will be there for a player like this, I told [club CEO Adriano] Galliani. Milan will set the price," the goalkeeper's agent said (h/t Football Italia). "The boy's contract won't expire. We just said we want to have the time to make the right choices."

Whatever happens there is unknown, but what brought Donnarumma to this point has been a combination of coaching and planning by Milan and the player's own hard work and dedication to his craft.

With the club slowly emerging from a desperately poor period in their history, fans of the Rossoneri believe his love of the club will keep him at San Siro for the rest of his career. So far he has been a beacon of hope in one of their darkest eras, but now they will dream of Donnarumma becoming the next great Milan icon.

Now that would be something to celebrate!