[Edi]

That is not a bug per se, but a function of how the different game mechanics work and the properties of different units.

To elaborate, it functions in the following manner:

Each unit is of a specific type, which is the monster ID number, e.g. 323 is Dwarven Smith. Each recruited unit also has an individual identification number which is not visible to the player, to differentiate from all other recruited units of the same type.

Each recruited/summoned monster shares all characteristics of its type. Some of these characteristics are fixed, some are variable. An example of fixed characteristic are the basic stats of a unit like attack, defense, immunities, base leadership values etc. Examples of variable characteristics are magic paths (even with commanders who have no randoms), experience etc that can vary between different units of same type. Current age is also one of them.

When a commander is resurrected as a mummy, it becomes a different monster. The mummy retains all variable characteristics of the original monster. They are tied to the individual unit, not specifically the monster type. Thus when type changes, it assumes the fixed characteristics of the new type and this causes the problem because the new fixed characteristic (maxage) is lower than the old one while the variable (current age) is retained.

This issue could be fixed by modifying the various mummy units to have a maxage of 10k, which would effectively cover all units in the game and eliminate the aging problem.