As someone who's cared for someone with a declining mental state, dementia patients should be able to leave whenever they want.

The problem is that the rest of the world doesn't want to - not can't, but finds it inconvenient - to deal with the ramifications of supervising or caring for people in public, strangers, who can't help themselves.

It's much easier to detain inconvenient people than change the rest of the world around them such that it wants to share the burden of caring for them.

Someone in my family, who is now in a memory care facility, has dementia. We tried to avoid it as long as possible, but when we got a call from the police saying that she was found wandering around barefoot in the snow next to a busy road in the middle of the night while hallucinating about “people coming to get her”, it felt like it was time to reconsider. She’s on meds that control the hallucinations now, but it would still be unsafe for her to leave unsupervised, no matter how much our culture changes. How do you help a stranger who doesn’t know where or who they are, and can’t hear too well to start with? She sure does still remember me and can dance to the Beatles, but I would not be comfortable trusting her welfare to a group of even the most compassionate strangers.

This is a very emotional topic so I want to avoid being too critical of that opinion, but at the same time, it's much much deeper than "doesn't want to". Dementia patients depending on their state are a danger to others and themselves. Their limited ability to understand what's happening around them means that they can get aggressive.

Even if not aggressive, they will just "get lost" and try to drive or "go for a walk" in the middle of the night in their pyjamas. I suppose from the way you phrased it that in your case it was mostly harmless and you just disliked getting weird looks, but a lot of dementia patients are actually incompatible with society and need to be detained or given anti-anxiety medications.

The general public is not trained to recognize or assist people with dementia. Allowing people with dementia to wander around city like NYC seems like a bad idea.

Many folks fully don't believe mental illness among younger people is as serious as it is. Somehow the frailty of old age legitimizes it.