LISBON • Renato Sanches is being hailed as Portugal's star of the future, but the dreadlocked wonder kid is already causing controversy amid the rave reviews.

"Portugal's prodigal son" and "the golden kid" were two of the headlines after the 18-year-old's stunning equaliser and calmness from the penalty spot kept Portugal on course for Euro 2016 glory, as they saw off Poland 5-3 in a shootout to reach the semi-finals on Thursday.

In doing so, Sanches broke Cristiano Ronaldo's record as the youngest scorer in the knockout stages of the European Championship.

But questions are being asked if Sanches is actually 18.

Born in the modest Lisbon suburb of Amadora, his Aug 18, 1997 date of birth has been questioned by several Portuguese sports pundits and, above all, by the directors of Benfica's age-old rivals Sporting Lisbon.

It is a controversy that refuses to go away.

Sanches has even threatened to sue ebullient Sporting president Bruno de Carvalho, who in March called on the Amadora hospital where Sanches was born to clarify once and for all the midfielder's true age.

Carlos Severino, a television commentator and a former candidate for the Sporting presidency, landed in trouble last November when he said Sanches' muscular physique was "very physically developed for his age" and suggested "he must have taken a lot of vitamins".

That description saw Severino slapped with an immediate claim for defamation.

The Jornal de Noticias newspaper conducted an investigation with supporting documents claiming the youngster was indeed born on Aug 18, 1997 at the Amadora-Sintra hospital, but the birth was not registered until Aug 22, 2002.

The report said that Sanches was barely five months old when his father, from the African island nation of Sao Tome and Principe, separated from his Cape Verdean mother and emigrated to France.

It was not until five years later that his father returned to deal with the registration of his son's birth, which is where the doubts surrounding his age arise.

In more controversy, his former youth club are accusing Benfica of failing to pay a promised bonus, despite the Portuguese giants cashing in on Sanches' talent by selling him to Bayern Munich last month for €35 million (S$52 million).

Affectionately called "Bulo", Sanches grew up in Musgueira, a densely populated area of northern Lisbon, where he started to play football at the age of eight for a local amateur club, before being spotted and recruited by Benfica when he was just nine.

Benfica made the modest promise of 25 balls and a bonus to the Aguias da Musgueira club should Sanches ever turn professional, but their president Antonio da Silva Quadros is still waiting.

"We don't have the balls or the money," he said.

The club are still seeking their dues from Benfica, while Sanches is headed for the Bundesliga next season.

AGENCE FRANCE-PRESSE