Here are my thoughts on what has been an interesting thread to read.
Would I discriminate against someone who was older when hiring a PhD student? Not at all, because I'd rather have someone who was dedicated and enthusiastic and knows what they want in life, than someone younger who was just doing a phd because they couldn't think what else to do, or because it was easier than getting a real job.
Do I think that a phd is necessary to publish? A phd per se is not necessary; I've met people who are very good at what they do and are valued by their research groups but simply never got around to compiling everything into a thesis. However, I think the mentoring and support of someone on the "inside" of academia is essential. Apart from anything else, publishing is quite political, and even if you're doing good work you need to know how to present the results so that it will survive the peer review process (which can be harsh). Of course you can stick to low-impact journals which will make it easier to publish, but then the chance that you will "make a difference" is correspondingly smaller.
I can only offer insight from a branch of physical sciences. Notwithstanding the opportunities available for amateurs in zoology and astronomy, in my field the days of the solo scientist working independently are long gone. We need powerful super-computers (expensive), and the input required to produce novel results is now too multi-disciplinary and large in magnitude for one person working alone to be able to do everything. Sometimes at our conferences there is a guy in his 70s/80s who is self-trained. He does publish things, somehow, but his models don't make any sense to an expert, and he's basically regarded as a bit of a joke in the community. That is why it is imperative to work with other scientists in my field - you just can't reach the same level of understanding without some supervision or interaction with colleagues.
I'm not saying that you can't do science on your own; I'm saying that if you want to make a difference publicly, you have to cooperate with others and be ready to play the politics.