for acne, the main reason is shukra and mangal. in years 15-26 (as you mention), the kam-bhava is at peak and this leads to a shukra imbalance in kundli. then, during this age, the harmonal changes take place which also turn out to be a reason for acne. anger and rash decisons lead to acne caused by mangal. frequently outing and snacking on foreign food causes disturbances in stomach which in turn force guru to give skin problems. last but not the least, the stress to excel, race for boyfriends and girlfriends, peer pressure and the state somewhere in between that of child and adult causes this problem.
I, too, have been facing the problem of acne for 4-5 years now.
use of neem, honey, cinnamon, rose water is recommended though these go only with time and patience.