Older stallions have lower quality sperm due to reduced mitochondrial function. Mitochondria are like batteries for sperm, giving them energy to move. The researchers studied 88 frozen samples from 19 stallions of different ages. Older stallions had less motile sperm and more damage from freezing. Better mitochondrial function meant higher chances of sperm moving well. Mitochondrial oxygen use increased until the stallions were 12 years old, then decreased. This shows that as horses age, their sperm quality and movement decline because of weaker mitochondria. Mitochondrial problems are a major cause of aging and freezing harming sperm quality.