This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax email@example.com with numerous contributions by others.
Schedules for people not vaccinated in infancy can be found, among
other places, in the Merck Manual and in AMA Drug Evaluations
Annual. There are two schedules, one for children under 7, and one for
people (children or adults) over 7. The reason is that pertussis
vaccine should not be given to anyone over 7. Pertussis is a mild
disease over the age of 7, but a serious one for the very young. For
that reason, the risks of the vaccine outweigh the risks of the
disease after the age of 7. It is possible that, with the availability
of a less reactogenic acellular vaccine, this recommendation may
change, and pertussis vaccine be give to older people as well, but
such a change will not occur without further study.