Most people recognize that a timing belt has a mileage limit, but do not know it has a TIME limit of 5 years.
Wrong, there has never been a "5 Year TIME limit" on TDI timing belts other than for inventory purposes at the dealership level.
TDI timing belts DO NOT USE "Rubber" but are completely synthetic compounds that do not oxidize like traditional rubber compounds.
TDI timing belt s REMAIN Mileage limited, and wear is a generally limited to side wear from centering on the pulley system.
The teeth of the belt and the non stretch internal chords (Kevlar) make "Stretching" a virtual impossibility.
Timing belt life can be greatly reduced by allowing "Road Debris" to get into the timing belt enclosure, this causes indentations in the belt and physical damage to the teeth which weakens the belt.
Cleanliness is paramount to a timing belts life!
Again, MILEAGE remains the ONLY limitation to service life.
VW at one time or another chose to limit shelf life to 5 years due to eliminating parts that may be superseded and subjected to updates and refined designs. This was to reduce the probability of old parts intermixing when superseded and newer designs being called for.
Gates has a good read on timing belts:
http://www.gates.com/~/media/files/...tomotive/timingbeltreplacementguide.pdf?la=en