Why is the 'drinking age' in the USA 21?

I am aware this question is not particularly thought provoking nor fascinating (or not for most I assume) but it has often inspired curiosity in me, perhaps simply due to my being raised amongst a different legal system (I have lived my entire life in the UK thus far) but nonetheless the curiosity is present. So why, may I inquire, is the legal age for consuming alcohol 21 in the USA? Granted there are obvious reasons such as the binge drinking issues in many areas of Europe (the UK certainly being no exception) but it occurs to me that firstly restricting the consumption of alcohol until such a late age is obviously going to provoke either illegal consumption or people who abide by the law until 21 and then cannot handle it neither biologically nor psychologically. Secondly, the age for smoking and joining the armed forces (dependant upon the particular branch) in the US is 18. People are expected to handle these responsibilities and make these choices at 18 yet cannot be trusted to enjoy an alcoholic drink without derailing?
I am exaggerating admittedly as there are evidently many bi-laws which overrule this law in various towns and counties however my fascination lies in the general conformity to the ‘21 law’.
Can anyone offer me some insight into this matter? Are there many protests against it? Does the law derive from a particularly ideology? Religious perhaps?