According to a post on Major Nelson, game demos and trailers are considered advertising, and are therefore subject to the regulations laid out in the ESRB's Principles and Guidelines for Responsible Advertising Practices. Xbox Live members under the age of 18 will not be allowed to download the material for any M (Mature) or RP (Rating Pending) games.
"ESRB guidelines are industry wide and are not unique to Xbox Live," according to the blog entry. "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 be in line with the ESRB's guidelines."
Feedback to the announcement has questioned why gamers can buy M-rated games from regular retailers at age 17, but cannot download demos for the same games via the online service. Information about how Xbox Live would conform to age regulations in countries outside North America was not given.