Maybe in USA. In Europe some countries even have special sentences for young adults (older than 18 less than 21). Here's for Germany:

"The maximum penalty for any crime committed by a person under 18 (or a young adult under 21 who is treated as a juvenile) is 10 years"

(Considering that people become more or less aware about their responsibilities and consequences of their actions at about 15 yo, he is now 4 times the responsible age he was at 16.)