Swampy, your post seems to imply he somehow "lost" his US citizenship. But to show that he did, you would have to have proof that he formally renounced his citizenship. And it would have to be him that did so, his parents could not have done it.
Maybe someone with more enthusiasm than sense heard something about the following and came up with the "18 years old" thing:
http://www.vkblaw.com/law/renunciation.htm
(quote)
Parents cannot renounce United States citizenship on behalf of their children. Before an oath of renunciation will be administered under Section 349(a)(5), persons under the age of eighteen must convince a U.S. diplomatic or consular officer that they fully understand the nature and consequences of the oath of renunciation and are voluntarily seeking to renounce their citizenship. United States common law establishes an arbitrary limit of age fourteen under which a child's understanding must be established by substantial evidence.
Under Section 351(b) of the Immigration and Nationality Act [8 U.S.C. 1483(b)], a person who renounced U.S. citizenship before the age of eighteen years and "who within six months after attaining the age of eighteen years asserts his claim to United States nationality in such manner as the Secretary of State shall by regulation prescribe, shall not be deemed to have expatriated himself...." The relevant regulation is Section 50.20(b) of Title 22 of the Code of Federal Regulations which requires that the person take an oath of allegiance to the United States before a diplomatic or consular officer in order to retain U.S. citizenship.
(unquote)
Clearly Obama would have had to have renounced his citizenship before he was 18 for the latter provision to have any bearing, but no one has provided any evidence that he did so, so the point is moot.