I feel like I wouldn't want to work with >70 years old professors because they're too old and have less chance of coming up with new ideas. is this a reasonable concern?
No. It is a bad idea to make assumptions about a person’s productivity and level of creativity, in the context of academic research, based on their age. This seems to me like an example of using the “fast” (aka System 1), heuristic-based type of thinking described in Kahneman’s book Thinking, Fast and Slow, to form judgments, instead of the more deliberative, slow, “System 2” mode of thinking. Kahneman describes many examples of this type of fallacious reasoning and the ways in which it can lead us astray in decision-making situations.
Each professor is an individual. Some make better advisors than others, but you’d have to estimate how good they are based on actual details about their personality, recent and less recent track record of success in research and in mentoring, and other relevant factors. Age can only be relevant insofar as it relates to someone’s physical health or an imminent retirement. Many academics in their 70s still produce excellent research and successful PhD students.
Don’t make assumptions, and don’t use heuristics in making important decisions like choosing a PhD advisor.
I'm quite a lot closer to 80 than to 70, but if you made a proposal to me that was in my lane, or accepted a suggestion that I made for a research project, I'd serve you pretty well. I have a lot of experience and with that, a lot of ideas.
However, I'm retired, so that would be a potential obstacle and we would need to get the university to make a decision. But even if I were not quite yet retired, that would be an issue. I also enjoy the fact that I currently have very little professional responsibility and can pretty much do as I please. Some of those things are actually fairly "academic" though.
I'd be useless to you outside my lane, of course, but that was probably always true, though I was able to give some guidance to people with quite different research directions. Not as advisor, but as a committee member.
Some of us old folk still have a functioning brain. Some of us still have an interest in field, even if we don't publish, ourselves, anymore. But it is an individual thing, and not one to make generalizations about.
It might be worse to have an advisor who was so young and active that they had no time to give you when you need it. Every case is different.
There are a lot of things that go in to making a good advisor. One of them is the attitude that you are an important person and that they will do what they can to extend your education and advance your career. That probably isn't age related, but it might be.
So long as an academic is not suffering from cognitive decline in old age (and I speak here of serious cognitive decline; e.g., dementia, alzheimers, etc.), I wouldn't think there is any reason why they would lack ideas for research. In fact, you would probably find that the opposite is the case --- by the time they are at the end of their career they have probably stocked up so many research ideas over the years that they have more paper ideas than they could possibly complete, giving their students plenty of possible topics they could take on as papers. I speak here as someone in my early forties, and I already find that the rate at which new ideas come to me is faster than the rate at which I can turn them into publishable papers. Consequently, I already have many many half-written papers and jotted down ideas, and I'm not confident I'll ever finish all of the ideas I've started.
It is possible that there might be some drawbacks to having an elderly professor as a supervisor, but I wouldn't think this would be one of them. An older professor will also have some advantages in terms of breadth of knowledge and experience, and I can certainly think of a number of professors in my own field who are now in their seventies who are absolute giants in the field; any research students would be lucky to have them as supervisors.
Finally, you should also bear in mind that research supervision usually involves a "panel" of multiple supervisors, and while one is usually the primary supervisor, there is also support from others. It is not unusual to have a mix of supervisors with different levels of age and experience (and academic level) and this will usually help ameliorate any disadvantages of younger or older advisors.
If an academic is in their seventies (or whatever advanced age is "too old") and feels that they can no longer do a good job supervising a research student, most likely they will just self-select out of the supervisory pool and only take on a supporting role on the panel, if that. Others will self-select out of supervision roles completely because they are close to retirement and they don't want to take on a multi-year commitment of seeing a research student through grad-school; they have earned the rest after all.
I can't comment if a professor that is older in age is necessarily less capable to be productive from a physical/psychological perspective, but an issue to consider are the regulations regarding mandatory pension after a specific age. Depending on the country of the professor, they may be forced to leave academia after a specific age (in my country it is 67).
Having your advisor leave academia for a pension in the near future may be detrimental to your future carrier for many reasons:
If they leave before you have completed your PhD, you will need to change advisors, which may or may not delay your diploma.
People that are going to exit imminently are likely to not be as motivated as someone who has potential for future promotion.
If they are in pension after you have completed your degree, and you are looking for a new job or a promotion in academia, they won't be able to participate in any department meetings that would create a position for you, or in electoral committees to actually have you elected/promoted, so you are likely to have fewer allies in these key roles. A younger person will be able to help you for many years after you complete your degree, if you maintain a good relationship.
I for one can understand your concern. BUT it also depends on your field, no?
In computer science I would trust an 80 year old professor who hasnt been working in the industry for 40 years less, than a professor who is 50 and still had some experience with rather modern technologies.
On the other Hand I believe a 80 year old professor of biology, psychology or medicine to be just as fit as the 50 year old professor. That is because usually professors still work in that field.
In the end it always depends on the person honestly. Had professors talking about how great they were in the 70s and are completely disconnected from the modern breakthroughs in that field, but had also professors in their 50s telling about how they worked in a really really big company just recently and worked with all new advancements but were just bad at teaching and I felt like they didnt even understand the new advancements.
Also had really great and up-to-date professors that were about to retire and it actually was sad when they retired as I would have loved to learn more from them.
Actually there might be significant advantages to working with an older professor. Consider that an older professor is more likely to be well-known and respected in the field. Reputation is a very important currency in academia. Your advisor's reputation could open doors for you. E.g.
- You want a job after you graduate? A letter of recommendation from a very well-known professor will carry much more weight than a letter from a new professor that no one has ever heard of.
- You need access to some specialized equipment? A well-know professor probably has friends at other universities or national labs that would let you borrow it.
- You need funding to complete some portion of your research? An older professor probably has many contacts at various funding agencies from over the years. A few phone calls might get your research funded. A new professor just getting started won't have as many contacts.
- You want to get your work published in a top tier conference or journal? An experienced professor knows what it takes to make that happen, whereas a younger one may not.
As the other answers point out, this will be dependent on the individual. Just because a professor is old, doesn't necessary mean they are well-known. Evaluate each specific potential advisor.