This is an excerpt from an FAA fact sheet:
16G seats: In 1988, the FAA issued regulations requiring that all newly developed transport aircraft use “16g” seats. Using a test dummy, these seats undergo dynamic testing and evaluation regarding injury protection. Similar to automobile crash tests, the FAA tests are designed specifically for the aviation environment. Previously, seats were designed and approved to a static 9g standard with no occupant injury criteria. Most transport airplanes were developed before 1988. However, Amendment 121-315, effective October 27, 2005, required that transport category airplanes in part 121 operations, certificated after January 1, 1958 and manufactured on or after October 27, 2009, must comply with the 16g dynamic standard.
We have been recommending that all of our clients update to 16g seats even if they are not required. We believe the extra expense is worthwhile for the enhanced safety of the passengers.