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We report new Chandra X-ray observations of the shell supernova remnant Kes 75 (G29.7-0.3) containing a pulsar and pulsar-wind nebula (PWN). Expansion of the PWN is apparent across four epochs-2000, 2006, 2009, and 2016. We find an expansion rate between 2000 and 2016 of the northwest edge of the PWN of 0.249% ± 0.023% yr^–1, for an expansion age R/(dR/dt) of 400 ± 40 yr and an expansion velocity of about 1000 km s^–1. We suggest that the PWN is expanding into an asymmetric nickel bubble in a conventional Type IIP supernova. Some acceleration of the PWN expansion is likely, giving a true age of 480 ± 50 yr. The pulsar's birth luminosity was larger than the current value by a factor of 3-8, while the initial period was within a factor of 2 of its current value. We confirm directly that Kes 75 contains the youngest known PWN, and hence the youngest known pulsar. The pulsar PSR J1846-0258 has a spindown-inferred magnetic field of 5 x 10¹³ G; in 2006, it emitted five magnetar-like short X-ray bursts, but its spindown luminosity has not changed significantly. However, the flux of the PWN has decreased by about 10% between 2009 and 2016, almost entirely in the northern half. A bright knot has declined by 30% since 2006. During this time, the photon indices of the power-law models did not change. This flux change is too rapid to be due to normal PWN evolution in one-zone models.