Do movie ratings in the USA not prohibit entry based on age?

A recent news article, and the subsequent discussion, led me to believe that in the USA, theaters do not prohibit entry to people under the age of the intended rating for a movie. Is this true? How does it work? 

Example: In the UK, an 18 rated movie (equivalent R) would prohibit people under 18 from entering, regardless of parental chaperons. 
In the USA, the MPAA provides a voluntary rating system for movies. [1] This system allows for general audience (G) ratings, parental guidance suggested (PG) ratings, and restricted (R) ratings, among others. [1] While many theaters follow MPAA guidelines, some do not. [1] For example, Regal Entertainment Group's policy for a Child's ticket is age 3 to 11. [2] Additionally, for a Rated R movie, a person must be 18 or older to purchase a single ticket, and a parent or guardian 21 or older is required to accompany and remain in the movie with any underage individuals. [3]
The Motion Picture Association of America (MPAA) film rating system is used in the US and its territories to rate a motion picture's suitability for certain audiences based on its content [1]. The MPAA rating system is a voluntary scheme that is not enforced by law; films can be exhibited without a rating, although most theaters refuse to exhibit non-rated or NC-17 rated films [1]. The ratings go from G for General Audiences, PG for Parental Guidance Suggested, PG-13 for Parents Strongly Cautioned, and R for Restricted [2]. Under 17 requires accompanying parent or adult guardian [2]. The body that assigns these ratings is the Rating Board, located in Los Angeles [4]. The Rating Board consists of eight to 13 full-time members and is part of the Classification and Rating Administration [4]. The president of the MPAA chooses the chairman of the Rating Board but has no say over the board's decisions [4]. The board views each film submitted for a rating, estimates the appropriate rating individually, discusses their thoughts as a group and votes on what rating the film should receive [4]. A rating can only be overturned by a two-thirds majority vote [4].