Their firm backing of Joseph Marie Minala came after it was alleged that the 17-year-old was in fact 41.
The Cameroon star joined the Italian team in December and has impressed in the youth team in the U20s championship.
But a Senegalese website said that Minala confessed he was turning 42 this August and had lied about his age in order to send money home to his family in Cameroon.
Lazio have threatened legal action against senego.net and the player’s agent has also stepped in to defend Minala.
Diego Tavano, told Italian newspaper La Gazzetta dello Sport: “He had a difficult time growing up. Talk to him and you’ll see he’s a 17-year-old lad in every way.”
Lazio have said they are happy Minala’s documentation is “absolutely legitimate”.
In a statement, the club added: “We denounce this attempt from hostile figures to throw a sinister light on this club.
“We reserve the right to take action against those responsible for the protection of the club and its players.”
Minala, who previously had spells at Napoli and Inter Millan, said: “I have read the alleged statements posted on the website in which it says I confessed my real age which was different to what was stated in my official documents.
“They are false statements attributed to me by people who do not know me.”
The real ages of several African stars, like Nigera’s Nwankwo Kanu and Jay-Jay Okocha, have been questioned in the past.
The youngster has now shut down his social media accounts after abuse over his age.
– Mirror Sports