He's listed at 17 years old. Here's a photo of him celebrating the goal. For comparison, consider the young Wayne Rooney, whose teenage exploits seem to be forgotten once he became a cog in the AIG-sponsored machine. Another visage comparison: AC Milan's emerging star Pato is 20.
So what's our point? If Macheda is 17, he's a mature 17.
UPDATE: His age is crucial in making him old enough to benefit from EU employment laws but young enough that Lazio couldn't sign him to a professional contract while they trained him.