There has always been talks in modern law about the right to die. Is this implemented recently? Some hospices I heard work with it. But what is you are not geriatric, just have other disabilities, physical or mental? Mentally disabled patients are usually homeless, but many would take advantage of a right to die practice against homelessness. But even if you are not homeless, you may want the right to die. For example a chronic forgetfulness, when you keep losing your wallet, coat, everything, every second day, could fall in the right to die category. How restricted is the right to die law these days? What do you know about it?
Getting back to this, I think the most important thing is the individual at the center of it. No one has the right to decide for another person who is capable of deciding for themselves.
This is why you're never too young to have an advanced health directive (a.k.a. a living will). AFAIK, anyone who goes into the hospital for a surgical procedure is asked to fill one out before the surgery (the hospital will have the forms), but in an ideal world every adult would complete one. Here's an example:
http://oag.ca.gov/sites/all/files/a...eAdvancedHealthCareDirectiveForm-fillable.pdf
Lots of lawyer words, but page 3 is the pertinent part. You get to decide this; no one else. Your doctor(s) and family members should be given copies, and they're obligated by law to honor your wishes.
That's how it works at present.
The two examples you mentioned - mental illness and dementia - are what I'd consider out of bounds. The person can be declared legally incompetent, but that doesn't give anyone else the right to decide to end their life...nor, IMO, should it.