Major Nelson, the Xbox Live Director of Programming, has received a few questions as to why Xbox Live members under 18 cannot access game demos or trailers on Xbox Live.
The answer lies with video game ratings organizations, like the Entertainment Software Rating Board (ESRB). Game demos and trailers distributed over Xbox Live are considered advertising and in accordance with the ESRB's Principles and Guidelines for Responsible Advertising Practices Xbox Live members under 18 do not have access to demos or trailers for a title rated M (Mature) or RP (Rating Pending.) ESRB Guidelines are industry wide and are not unique to Xbox Live. In other words, these are the rules that all online console stores, like the Wii Shop Channel and the Playstation Store, need to adhere to in order to in be in line with the ESRB's Guidlines.