The event that happened last among the options provided is Title IX forbids gender discrimination in sports, which was enacted in 1972. The other events occurred prior to this:
- The YMCA opened its first gym in America around 1851.
- Physical education became part of American school curricula in the late 19th century.
- The adjustable plate-loaded barbell was invented in the 20th century, but specific timelines can vary.
Therefore, the correct answer is Title IX forbids gender discrimination in sports.