One thing to point out. Age is an odd thing in the DnD universe. A mortal can be virtually immortal.
I suspect the Emperor was actually an Ulitharid (he either never showed his extra tentacles or it was an artistic oversight). IIRC they can live much longer than a basic mindflayer, if properly nourished, and can become an elder brain if allowed. This IMO is the Emperor’s ultimate goal to ensure his freedom (enact the grand design with himelf as the nether brain 🧠once he “consumes” the actual nether brain). Only strong servants may serve him, the rest are fodder.
All the Emperor’s abilities (including shifting in and out of the prism) is explained by this. See the wiki below to compare. Your charecter can’t go in and out of the prism even though you possess it.
https://forgottenrealms.fandom.com/wiki/Ulitharid
