There is never a right time to seek help from a professional, we are all individuals. Like our various patterns of drug use, our patterns of seeking help vary.
In general it is an awareness that is raised, your family start to make comments about your alcohol use for example. Your first in the bar and last out, In fact your partner might be in the process of leaving you, your work attendance is getting worse, you might be facing financial difficulties and a range of medical issues.
If you inject drugs, you might OD (overdose) your crime rate increases, you have periods in jail. Stealing and or lying become second nature, your family and friends might distance themselves. If you IV you might use legal highs or any available substance, you take more risks.
If you use ‘new drugs’ that you have purchased on the street or from someone you know, these might be ‘white powders’ or claim to be ‘ecstasy’ you find yourself using all weekend, no sleep, it’s party time, you then sleep all week, you miss school or work. Your grades are affected. You start using on your own, you start to loose your temper more, you feel ill, the hallucinations are less fun, in fact you fear them, You can go on-line http://www.cool2talk.org/ or Crew https://www.crew.scot/
You can contact services for young people.. https://www.pkc.gov.uk/article/17901/Services-for-young-people
Basically there is no one route, you will decide because of a few factors, but once you decide you will start a journey that most people find positive.
Please watch the TED Talk by Gabor Mate which you might find really interesting about some of the why’s people use substances and other addictive practices.