A young boy or girl who gambles at the age of 18 can define what they want and how they can get it. Someone who's truly 18 years old can live and be able to provide for themselves to some extent, depending on the country.
Why the age of 18 and above is placed on gambling because one has learned how to take responsibility for their actions. They must have learn how to survive on their own that whatever they do, there's a repercussions for it.