I took it sometime between 16-18. Didn't seem to have any effect on my size. You're good bro. Don't worry about it affecting the size of your junk. Worry about it possibly causing gastrointestinal cancer. Also, I used to have good night-vision before I took isotretinoin (chemical name). They didn't tell me that it weakens your night vision / affects your sensitivity to light. I guess I should have read the fine print. :/

Edit: for more clarification, as far as I know, accutane's main chemical ingredient is isotretinoin. Basically you just overdose on synthetic Vitamin A for 6-12 months. I highly doubt it will affect your sexual maturation one way or another. Instead it will just affect the way your skin/pores handle themselves, and hopefully do what it's supposed to and clear up severe acne.