In Jain time‑cycle, the 4th āra is Dukhama‑Sukhamā. Scriptures don’t give one “common/average” age for this whole āra, because the human lifespan steadily decreases throughout it.

• At the start of the 4th āra: the maximum human lifespan is 10 million pūrvas (an unimaginably long time).
• As the 4th āra progresses: lifespan keeps decreasing.
• By the end of the 4th āra: it declines to the range that leads into the 5th āra, where the maximum is about 120–130 years (Shvetāmbara texts usually state ~125 years; many Digambara sources mention ~130 years).

So, there isn’t a single “common age” for the 4th āra—only a very high maximum at the beginning that continually reduces through the era. For a simple overview of āras, see this note on Jain time from JainKnowledge .