There's nothing on the Federal Aviation Regulations specifying age, but I'm sure that in the US there's a bevvy of other laws regulating it.
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Section 121.575: Alcoholic beverages.
(a) No person may drink any alcoholic beverage aboard an aircraft unless the certificate holder operating the aircraft has served that beverage to him.
(b) No certificate holder may serve any alcoholic beverage to any person aboard any of its aircraft who --
(1) Appears to be intoxicated;
(2) Is escorting a person or being escorted in accordance with 49 CFR 1544.221; or
(3) Has a deadly or dangerous weapon accessible to him while aboard the aircraft in accordance with 49 CFR 1544.219, 1544.221, or 1544.223.
(c) No certificate holder may allow any person to board any of its aircraft if that person appears to be intoxicated.
(d) Each certificate holder shall, within five days after the incident, report to the Administrator the refusal of any person to comply with paragraph (a) of this section, or of any disturbance caused by a person who appears to be intoxicated aboard any of its aircraft.
[Doc. No. 6258, 29 FR 19219, Dec. 31, 1964, as amended by Amdt. 121-118, 40 FR 17552, Apr. 21, 1975; Amdt. 121-178, 47 FR 13316, Mar. 29, 1982; Amdt. 121-275, 67 FR 31932, May 10, 2002] </font>
However, anyone who's traveled outside of the US knows just how silly an age-based rule is!
[This message has been edited by hillrider (edited 11-26-2002).]