Correct way to say "when I was young..."
You can say:


  
  当我年轻的时候，...
  我年轻的时候，...
  年轻的时候，...
  年轻时，...
  


In these cases, you are talking about the time when you were a young man (note: not a kid nor a teenager). Perhaps you are now in your 40s/50s/60s etc, and you are talking about your 20s/30s.

You could also say:


  
  当我年幼的时候，...
  当我还小的时候,...
  我年幼的时候，...
  我还小的时候，...
  我小时候，...
  小时候，...
  


In these cases, you are talking about the time when you were a kid or teenager.