As per available Jain scriptures and traditional Jain literature, the exact age of Keśi (Keshi) Gandhar at the time of his death is not clearly mentioned.

We know about:

• His role as a prominent Gaṇdhar of Bhagwan Pārshvanāth (in some traditions, identified as Keśi Śramaṇ / Keśi Āchārya).
• His famous dialogue and meeting with Gautam Swami (Gautam Gandhar), where they discuss the differences and essential harmony between the teachings of Pārshvanāth and Mahāvīr Swami.

However, specific numerical details like his age at death are not preserved (or at least not commonly available) in standard Jain scriptural tradition, either in Digambar or Shvetambar sources that are normally referred to.

So the honest and authentic answer is: We do not have a reliable, scripturally supported number for the age of Keshi Gandhar at the time of his death.