As tar ages, it can lose lighter compounds, harden, dull, crack, gather dirt, darken the surface beneath it, or become less protective.
Tar ageing is not one process. Fresh tar may be glossy, wet-looking, smoky, and tacky. Over time, lighter fractions can evaporate, oxygen can affect the surface, dust can stick, and weather can wear away exposed layers. Sun, rain, salt, frost, and movement all change the result.
On wood, ageing can be visually attractive. A tarred board may settle into a dark, matt, weathered tone. On rope, ageing may mean stiffness and loss of flexibility. On roofs or seams, ageing may become failure: cracks, gaps, brittleness, or loss of adhesion.
Traditional tar use usually assumed maintenance. A pine tar finish could be renewed rather than abandoned. This makes it different from some modern coatings that are expected to form a sealed film and then be replaced when they fail.
The important distinction is between appearance and function. A surface may still look dark and tarred while its water-shedding ability has declined. Old tar should be judged by condition, not colour alone.