I wouldn't go as far as a tattoo - something closer to an allergy bracelet would be a less invasive solution.
You also should consider the degraded skin of the elderly, the dangers of anaesthetising them, and the difficulty of putting (potentially very uncooperative) dementia patients through painful procedures.
There's also lower complexity community wide solutions like fake bus stops in and near nursing homes - it may not be a catch all, but it's nice mitigation for an unresolvable issue.
You also should consider the degraded skin of the elderly, the dangers of anaesthetising them, and the difficulty of putting (potentially very uncooperative) dementia patients through painful procedures.
There's also lower complexity community wide solutions like fake bus stops in and near nursing homes - it may not be a catch all, but it's nice mitigation for an unresolvable issue.