I would think that most dementia is caused by physical failure of the brain, not some kind of kammic destruction of the mind. So if one is not in control of their actions or otherwise capable, then it would be difficult to argue that they would suffer in the next life for it.
Gain and loss, status and disgrace,
censure and praise, pleasure and pain:
these conditions among human beings are inconstant,
impermanent, subject to change.
Knowing this, the wise person, mindful,
ponders these changing conditions.
Desirable things don’t charm the mind,
undesirable ones bring no resistance.
His welcoming and rebelling are scattered,
gone to their end,
do not exist.
- Lokavipatti Sutta