The U.S. Senior Open is one of the five major championships in senior golf, introduced in 1980. It is administered by the United States Golf Association (USGA) and is recognized as a major championship by both the PGA Tour Champions and the European Senior Tour. The lower age limit was 55 in 1980, but it was lowered to 50 for the second edition in 1981, which is the standard limit for men's senior professional golf tournaments. By definition, the event is open to amateurs, but has been dominated by professionals; through 2022, all editions have been won by pros. Like other USGA championships, it has been played on many courses throughout the United States. Allen Doyle became the oldest U.S. Senior Open Champion in 2006, winning two weeks before his 58th birthday. The total purse was the highest of any senior tour event until the Posco E&C Songdo Championship in South Korea, a Champions Tour event in 2010 and 2011 with a $3 million purse, but had a lower winner's share ($450,000). The U.S. Senior Open is again the highest purse on the PGA Tour Champions; in 2016 it was $3.75 million, and champion Gene Sauers earned $675,000. The purse in 2017 is anticipated to be $4 million, yielding a winner's share of $720,000. Like other senior majors, players must walk the course unless they receive a medical exemption to use a cart. Winners gain entry into the following year's U.S. Open. The playoff format was modified for 2018, reduced from three to two aggregate holes, followed by sudden death. The three-hole aggregate playoff was used in 2002 and 2014; the final 18-hole playoff at the U.S. Senior Open was in 1991, won by Eligibility The following players are exempt from qualifying for the U.S. Senior Open, provided they are 50 years old as of the opening day of the tournament. Amateur categories require that the player is still an amateur on the opening day of the tournament, except for the one-time exemption for former champions of the U.S. Amateur or The Amateur Championship. Any past winner of the U.S. Senior Open Winners of any of the major championships in the last 10 years Winners of any of the U.S. Amateur in the last 10 years and runner-up in previous year Winners of the Senior PGA Championship in the last 10 years Winner of the Senior Open Championship in the