Sometimes, time will help clear your questionable past.  Had I tried to get a clearance immediately upon joining the Armed Forces, they would have laughed me out of the office.  However, after six years of serving in the Armed Forces prior to crossing over to a field that required a TS/SCI, the time I kept my nose clean seemed to diminish the damage from my less than brilliant teen years with offenses that included marijuana use and a minor in possession of alcohol.

About 4 years ago, I was recruited by a defense contractor and given a CIA poly, which I failed due to admitting that I had concealed marijuana usage - on my form I had admitted to smoking 4 times, and in the polygraph, I admitted to smoking several more times.  Yes, I know, dumb, but I was young and naive.  The letter of rejection stated that this was the cause for my unsuitability.  It was devastating, and even moreso because, in the D.C. area, it seemed that I was turned away from job after job - even when people admitted that I was the only qualified applicant - because of this experience.  Since then, I have obtained a Master's Degree in Arabic Linguistics from Georgetown, and have obtained about 3 years of experience as a professional linguist, specializing in machine learning, translation, and localization.  I also speak Mandarin and Spanish, and I can read several other languages, including Farsi and Urdu.   

I do not have any pressing need to get a security clearance, or to have anything to do with the military or the intelligence community - I have a good job in the private sector now.  However, I keep reading articles about how people with my skillset are badly needed in our struggle against terrorism, and it feels bad to be on the sidelines.  Is there a chance in the distant future of recieving ANY kind of clearance at all, or should I give up the idea entirely?