You can use any amount of money to invest where you do not have to mention any specific amount. You can use any amount starting from a minimum of one dollar per day or per week. Also, there is no specific mention here that Bitcoin can be used under the age of 18 or not. But of course I will say that by investing, you have to keep that investment safe. If it is possible to keep it safe, then it is better to invest and if it is not possible to keep the investment safe, then the investment will be very logical and you can lose at any moment. You start your journey, but definitely have a long-term plan to use DCA, I think you will definitely be very successful.
When it comes to investing in Bitcoin, you can use the fact that you can invest any amount, as little as you can invest. this is more reason why a lot of interested in bitcoin because you can buy any amount because that is one of the priorities, but it wont be reasonable for you to be investing even 1 dollar into bitcoin because what is now the relevance of investing so there is more reason to want to invest big than investing bitcoin.
And when it is a logical situation, you can make plans on how to buy because when you are serious about your investment, there are a lot of things that should make you consider investing because you have to make plans. and it might not go the way you want first, but even when it does not go your own way, you can just try harder because this situation can make you to be discouraged, but it is better not to give up.