Antonie "Hunter" Farani was 14 years old at the time of the murder.
Prosecutors say Farani and three other people kidnapped 18-year-old JoJo Brandstatt and shot him at the West Ridge Golf Course in February 2009.
A jury convicted 21-year-old Spencer Isaiah Cater in December 2011 of felony aggravated kidnapping and aggravated robbery, while acquitting him on a murder charge.
Two other people pleaded guilty for their roles in the crime and were previously sentenced to 15 years to life for their roles in the crimes.
Prosecutors say Brandstatt was marched at gunpoint onto the course, shot and left for dead.
Police said that then-14-year-old Farani confessed to pulling the trigger but claimed he was high on marijuana and pressured by Cater.
Farani's case was tied up in appeals over whether he should stand trial as an adult.
He pleaded guilty in March.
In court Friday, Farani was sentenced to 20-years-to-life in prison on a single 1st Degree Felony Aggravated Murder charge, while receiving two 15-years-to-life sentences for two separate felony kidnapping charges.
The murder sentence will be served first, followed by the kidnapping sentences, which be served concurrently.