Is it normal or even helpful for teenagers to still be eating flintstone vitamins?

Yes. It is normal and somewhat helpful for teenagers to take a daily multivitamin. Teens often don't get enough vitamin d or calcium in their diets. If they are choosy eaters and don't eat a variety of foods, they may be low in other vitamins. A teen is usually big enough to take an "adult" multivitamin, but it is fine for him to take several kid's chewables instead (but he'll be chewing more sugar).