If adults work is lighter, then continuing it shouldn’t be a problem in that case.
So a very powerful device is needed to be an elder and I like that because an attack to get control of a section is harder to implement.
Furthermore, I would find unfair that any requirement for elders be less constraining than for adults. It could be storage (capacity and speed), cpu, download, upload, …
I wouldn’t be alone, and the risk is a perceived injustice damaging for the network success if the requirement could be lighter in one of the dimensions (say for example storage capacity).
The only way to ensure this is that elders work is additive to adults work. If the work is different we can’t be sure of that or at least it might be difficult to convince people of that.