Between the Ages of 12 and 14
Nutritionally, vitamins marketed for children and adults are similar. The main difference between them is that children's vitamins are typically chewable and fruit flavored, while adult vitamins are swallowed. Experts recommend switching from children's to adult vitamins when the adolescent is able to swallow pills. This is typically around ages 12 to 14.
Additional Calcium May Be Needed
Though adult vitamins are similar to children's vitamins, the main nutritional difference is calcium. Children need more calcium than adults. When switching to children's vitamins, adding extra calcium through an additional supplement or changes in the diet will ensure the adolescent receives his daily requirement of calcium.
A boy should switch to an adult vitamin when he is able to swallow them whole, which is usually between the ages of 12 and 14. He should also either take an additional calcium supplement or incorporate the recommended dosage of calcium into his daily diet.