Tar ages by losing lighter compounds, oxidising at the surface, collecting dust, becoming duller, and slowly changing from a sticky or glossy coating into a harder weathered layer.
Fresh tar is usually more mobile, smellier, and glossier than old tar. Over time, lighter fractions evaporate, the surface reacts with air, and sunlight changes the outer layer. A coating that once looked wet can become matt, brown-black, grey-black, cracked, or dusty.
Ageing is not always failure. Many traditional tar finishes are expected to weather. A timber wall, fence, or boat fitting may be recoated periodically, with each new layer adding to a dark maintained surface. In that setting, ageing is part of the maintenance cycle rather than a surprise defect.
The rate of ageing depends on exposure. A sunny south-facing board may change faster than a shaded beam. Rain can wash away weak surface material, salt can leave deposits, and heat can soften the coating during the day before it firms again at night.
Old tar should be judged by behaviour rather than age alone. If it still sheds water, remains bonded, and protects the surface, it may simply be weathered. If it has cracked, peeled, powdered, or trapped moisture, it needs closer attention.