I’m not sure if anything has been confirmed, but this was my take away from the livestream-
Spellcasters have a magic meter separate from the Energy bar. The more they use magic, the meter fills. This is good to a certain extent, because as they fill up with magical power, they become more powerful. But raw magic is dangerous, and if the meter fills too high they risk losing control of their powers which will lead to death. This effects all spellcasters, no matter how old they are. A familiar can save them, and there are perk points to prevent this from happening (I think).
Elders have always been at risk of dying from over exertion. This can happen if they woohoo too often or exercise too much. This has to do with their Energy bar. I don’t think that over exertion and magic are connected. Two separate things, two separate deaths.
Maybe you would find your elder more likely to become over exerted from dueling, I’m not sure if that will count as a strenuous physical activity. But I would assume elders can still be very powerful spellcasters despite their age.
Again, just my impression from the livestream. I’m not positive on how it’ll work ?
EDIT to add: there’s always the immortality potion to make sure your sim stays safe ?