is it illegal to smoke cigarettes under 21
The federal law requires states to have a minimum age of 18 years for sale/purchase of tobacco products (21 U.S.C. § 387f. (3)(a)(ii)). State laws may extend this ban and also prohibit supply, possession and consumption to/by person underage. But most of the states do not directly ban the consumption of tobacco by a minor. The federal law requiring states to have a minimum purchasing age at 18 is enforced by withholding FEMA disaster and non-disaster grants from states that have purchasing ages under 18, and currently no state falls under that category.
Smoking age