Unfortunately I think Joruus's age might be a bit of a continuity snag. At the time Zahn assumed the Clone Wars were started much earlier in the timeline and thus probably also assumed that the Jorus clone would have been created much earlier. However, under the assumption that the clone was aged to maturity using Spaarti cylinders in a 1 year time span, Joruus could have been physically equivalent to about 45-50 (20-25 rapid aging years + 25 real years). Perhaps he was rapidly aged even further as a plan to replace to original C'Boath. It has been too long since I've read the Thrawn trilogy to recall all the details.