It kind of varies with each jurisdiction and country, when we talk on the age which could be proper for a person to start to gamble. For example, in the United States people as young as sixteen years old are considered to be capable of having a job, driving a vehicle but not to smoke or drink alcohol. If someone is already mature enough to drive around and keep a job then there are other different reasons why they are kept from drinking and gambling. Again... it depends on the country/State.
In general, driving and having a job are qualities which are inherently considered to be positive for the life of an individual within society, whereas gambling is highly stigmatized and frown upon in most cultures. In theory, in the case of the USA, teens care capable of getting their own economical resources.