>Age isn't an issue when all parties are adults.

there's exceptions to every rule but as a general statement that's about as false as it gets. With increasing age gap between partners divorce and breakup rates go up significantly. Cultures with strong aversion to age gaps, East Asia for example, have both low divorce rates and out-of-wedlock births.

The reason isn't extremely difficult to see, where someone is in life, what priorities they have and how responsible they are is significantly influenced by age, the rom-com industrial complex might have convinced people that relationships are about butterflies in the stomach, but in reality compatibility matters.