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The correct choice highlights the fact that a high rate of sludge wasting directly influences sludge age by reducing the amount of biomass retained in the system. Sludge age is essentially a measure of the average time that activated sludge remains in the treatment process, and it is crucial for optimizing biological treatment processes. When sludge is wasted frequently or in large quantities, the biological population has less time to grow and mature, which can lead to a reduced sludge age.

In contrast, the other options don't directly contribute to a shortened sludge age. For example, plenty of biogas production indicates that the biological processes are active and functioning well, which generally doesn't correlate with a decrease in sludge age. Stable process conditions suggest that the system is operating uniformly, which typically supports sustained sludge retention rather than a decrease in sludge age. Lastly, low organic loading tends to slow down the growth of microorganisms, effectively extending sludge age because there is less demand for biological activity and sludge turnover.

Thus, a high rate of sludge wasting actively shortens sludge age by minimizing the time biomass spends in the system, making it the correct reason for a reduction in sludge age in an activated sludge process.