As the comments suggest, "too young" is not a protected class.
As another example of this principle, most car rental companies (if not all) will not rent to anyone under 25 years of age (regardless of their driving record or insurance status). This is not a legal restriction, but a standard industry practice.
Companies can legally claim that they believe any kind of fiction about their customers if it's not explicitly illegal to do so. If they think that being below a certain age makes a person too young to have a good judgement, they don't need to be correct about it. They can act on that opinion (and lose business) as they please.