The admission means Mohammed Ajmal Kasab, a Pakistani national, can face trial on 86 charges including waging war against India, murder and attempted murder, CNN reported. The court already had rejected his underage claim.
Defense attorney Abbas Kazmi said Kasab actually was 17 ½ and gave his age to a judge as 21 only because the court believed he was not a juvenile.
Kasab is one of 10 Pakistani nationals who police said unleashed terror on Mumbai, India's financial capital, for four days and three nights in November.
The gunmen were accused of killing more than 160 people as they took over three luxury hotels and a Jewish center, and shot up several other places.