As a teenager and an adult, Goten is still kind-hearted and considerate of other people, but is seen to be more mature. He settles down to live more or less as a.
Adult Gotenks is the adult version of Gotenks. He is the fusion of Goten and Trunks by Fusion Dance. Adult Gotenks is a lot more powerful compared to his.
Goten (孫悟天 Son Goten) is the second son of Goku and Chi-Chi and the they disguise themselves and enter the Adult Division, as Mighty Mask and are later.
By the same logic, why is Trunks an adult in GT and not Super? Why is Pan an In Dragon Ball Z, why is Gohan born with a tail, but not Trunks or Goten?.