It all depends on your states laws some allow under the age of 21 in the bar and if you live in a place like Texas I believe, some places will even allow them to drink with their parents consent. Like I said It all depends really.
As for the should they. Well most people say if your parent drinks you will drink and I say Nay, my parents let me taste beer when I was younger and as most people first trying beer I did not like it and spit it out. I never tried it again and even now even though I do drink it is in moderation with self control. If you teach them at a young age what it is and why it is dangerous it can help them to not make the mistakes that so many teens make when they brake out into the party scene around high school.