https://www.associatedcontent.com/article/86691/what_is_the_legal_drinking_age_in_the_pg2.html?cat=17
said:
It was determined by several studies that a youth's brain is not fully developed until about 21 years of age, and that because of that, alcohol affects youth's brains differently than it does an adults. That magic number '21' kept coming up all over again, and eventually, the Act was passed requiring states to increase the legal drinking age to 21 or risk losing a percentage of funding for their highways. The Act gave them two years to do so. Some states did; some states didn't – but currently, all states have enacted some type of minimum drinking age legislation, with certain exceptions being allowed by at least 31 states for parents providing it, entering establishments (for example, in Texas, you must be 21 to drink, but many bars allow you to enter the establishment if you are 18, you just are not allowed to drink while there), and other exceptions. Of course, the 21 year old drinking age minimum requirement does make exceptions for religious ceremony and medical purposes too.