Anyone can decide to own investment in bitcoin even if they are below 18 years because they are basically investing in their future so far as they have the discretionary amount to do so and if they have already started earning money on their own. An 18 years old is considered to have approached an age that they can take their own decision without their parents interfering with their decisions so yeah at that age they are good to go with their investment in bitcoin. 
      @reefsea should have said that those who are 18 years and above can invest 10% of their income instead of saying $10 because at 18 years, they don't have much responsibility to spend their money on and the amount the will invest will be determined by the size of their income, so within that age is the time of building their future by investing their money rather than spending lavishly. At that age is a time to figure out and make decisions that will clear the path for them in the future because not too long they can enter into marriage and they become tied to responsibilities such that they may not have enough to invest again and also take care of their family so the earlier will be the better for them at that age.
      No one starts an investment without a source of income because even for someone who want to invest using the DCA strategy it will still require a source of income before you can confidently be funding your DCA at regular intervals.