I would probably leave out any drug use experience.
Even in the field you are going into....you are still a prospective employee.
It sounds like you have a great deal of life experience in lifting yourself out of hardship, taking responsibility for your life, and achieving many of the things people who seek help for drug problems and addictions strive for.
I congratulate you - it sounds like you've walked a very hard road in life, and found your way out of enormous difficulty.
That's really impressive - not just on a personal level, but also as an applicant/employee - you've shown ambition, strength of character, the ability to solve problems, use initiative, make things happen. Employers look for all of those things.
Now, as for the "drugs thing" - I think you've given a very good example here;
You have witnessed your friends go through phases with various forms of drugs.
It is also highly likely (in most people's minds, at least) to have witnessed drug use, known drug users and developed an empathy for people suffering from addiction - in your period of homelessness.
Whether there is any truth to this perception or not, even homeless people that are sober are presumably likely to encounter drug use - whether it is knowing people that use heroin, alcohol or whatever else; having been homeless doesn't mean you are or have been a drug user or addict, and in the context of a drug counsellor, it is this experience (that you've not led a sheltered life, essentially) that may be worth mentioning.
But generally, when it comes to job recruitment - your private life should be of little to no consequence, so long as you meet all of the other necessary requirements.
Depending on the job, having a drug conviction on your criminal record may be an exception to this, but that not being the case, it is really not relevant to employers.
In the unlikely case that you are directly asked about your own drug use (which frankly is none of any employer's business - unless you already work for them and your substance use is impacting upon your performance at work) I would advise disclosing as little as possible.
Do not bring up the subject of your drug experiences (as a general rule - i can't really imagine too many jobs where personal experience with illicit drugs is sought after by the employer - even in a head shop or working in supervised injecting facility, any such experience is more or less implied - or simply not relevant).
One of the important things to remember in dealing with potential employers is to focus solely on positive attributes you can offer the organisation (I have been looking for work lately, so the 'job search' element is still at the forefront of my mind - and honestly, once you have a job as a drug counsellor, nobody is going to judge you for being young or potentially taking drugs yourself - I attend weekly drug counselling sessions, and the appropriate atmosphere of such workplaces is a non-judgmental one [at least it should be - and is in my case!]).
In this case, I would simply recommend emphasising your professionalism, your skills, your personal work ethic and - if it seems appropriate - triumphs over adversity and hardship.
Keep in mind that this is a job like any other, ultimately.
If an employer enquires about your motivations for going into this field - you can tell them of your desire to help clients/patients and demonstrate your empathy towards people in their position.
If anything, your experience witnessing and knowing about drug use may be worth mentioning (but only if it comes up in conversation, IMO) - but it is worth remembering that many people go into this and similar fields with little or no prior experience with drugs, but simply because they care about helping people and contributing to society in their work life.
It is a respectable and noble career choice, in my way of thinking; personally, my drug counsellor has helped me immensely in getting perspective on my life and my drug use, and this has improved my quality of life and been immensely helpful in times of stress or temptation.
But I would definitely not recommend discussing your own drug use - it shouldn't come up in a professional setting (unless you are going to work visibly intoxicated) and it would be unprofessional to be asked (in my experience, anyway - I do not, and have never worked in an industry or a country that regularly drug tests employees).
As for the age thing - again - well done for having achieved so much by the age of 21.
Think of it as a positive attribute, not something to be self conscious of.
There may be a correlation in your thinking between 'youthfulness' and 'drug-taking' (if I'm reading you correctly) - but as a guy 9 years older than you; trust me - age and drugs have no particular link. I know 60 year olds that take drugs and 24 year olds that don't.
It sounds like you'll be in a great position to be a knowledgeable and fully competent counsellor.
In a professional sense, "life experience" is rarely taken into account - credentials such as study and professional experience is far more applicable (in whatever form that takes for the job yoi want to do; if that means volunteering at a rehab centre or something pf that nature, it may be far more relevant from an employer's perspective).
However, it sounds like you not only have relevant life experience (in hardship) but also the incredibly valuable skills to have worked your way out of a bad situation - and at such a young age.
Sounds to me like you have nothing to worry about, and that you could do a lot of good and be able to help people who really need it.
On a practical level (ie one between you and a patient - not the boss) - your knowledge and understanding of drugs could be a great bonus, rather than a hindrance. Understanding and empathy is really important in counsellor/patient relationships, and I can't see that being a problem.
All the best!
