1900, per NBC's website.
cross country show jumping and dressage
London Olympics
Three Day Eventing was first introduced to the Olympics in 1912, with women being allowed to compete in 1952 - note that the Equestrian events are the only ones where women are allowed to compete against men on equal terms
To win the Equestrian you have to have the most points for dessage,jumping and eventing.
Day 4 of the 2012 Olympics was when NZ got their first medal. it was a bronze won by the NZ equestrian team.
Cycling has been competed at the Olympics since the first Modern Olympics in Athens in 1896.
Equestrian.
the Olympics (eventing)
Equestrian
Galen Spencer was the oldest to receive equestrian medal.
Partial answer: The ages of the riders representing the United States in Beijing are given with their bios on the website 2008 Olympics Equestrian Cheat Sheet, http://www.theequinest.com/2008-olympics-equestrian-cheatsheet/
Swimming was first included in the Olympics in 1896, but has only been open to women since 1912. Hope this helps.