The o.p. was in regards to a family monitoring a demented family member.  Didn't say anything at all about whether the family member with dementia agreed to it or not.  Last time I checked, individuals still have some rights until they are 'declared' unfit in some way or other. 

At what level of dementia shall this start at?  The first time they forget what they walked into a room for?  When they forget to fill their vehicle full of fuel and it runs out?  When they forget to charge the battery on their phone? 

Who sets the standards for declaring the level of dementia one must suffer from, before allowing family members to start making choices for you?

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There's a difference between an adult person voluntarily wearing a device to be monitored by others and someone forcing them to submit to monitoring. 

The point at which someone crosses the threshold of needing monitoring vs. not needing it will vary in the minds of many.

Some parents would probably like to force their 20-30 year old kids to wear a monitoring device so they could know where they are.  Most 20-30 year olds would resist I think.  If the parents could declare them demented, what would stop them.