At your place I wouldn't do 16 weeks for a first cycle... 23 years old is a bit young so the longer you shut down your HPTA the harder it will be to recovery. So play safe and stick to 12 weeks.
You can include clen during PCT. It will help you to keep some gain due to the anti-catabolic effect of the clen.

almost 40 pounds great jobs. I hope you'll be able to keep a good part.