It is unlawful for any person under the age of twenty-one years to acquire in any manner, consume, or have in his possession any intoxicating liquor; provided, that the foregoing shall not apply in the case of liquor given or permitted to be given to such person under the age of twenty-one years by his parents or guardian for medicinal purposes pursuant to the written prescription of his physician or dentist, or administered to him by his physician or dentist for medicinal purposes.