Simplify the following text for a Grade 8 native English-speaking person.
Original: He spent six months in a juvenile detention center after a robbery and later completed 9 Â 1\/2 years of an 18 Â 1\/2 year prison sentence for armed robbery and kidnapping which at the time he started it he was sixteen .
Simplified: He spent six months in a juvenile detention center after a robbery . He later completed 9 Â 1\/2 years of an 18 Â 1\/2 year prison sentence for armed robbery and kidnapping which he started at the age of sixteen .