A man who is advanced in years ... must not be appointed member of any Sanhedrin, since he is required to be compassionate. Hil. Sanhedrin 2.2
This is based on several Mishnaic (Horayot 1.4) and Talmudic (Sanhedrin 36) sources (see here). Rashi, for example, vaguely mentions "forgetting the suffering of child upbringing".
At what age is a judge too old and incompassionate to function in the Sanhedrin?