How should I take a potential PhD supervisor's age into account, when planning to follow PhD with habilitation? I plan to apply for PhD in Finance/Statistics next semester, and hence searched for potential supervisors.

An issue I have is that many interesting candidates are above age 60, and in Germany Professors retire at 65, such that a subsequent post-doc/habilitation would most likely require a different supervisor after PhD. One Professor just started his position at age 40, but so he has not many notable publications and I am unsure whether he might change university soon (he just changed it from another 4 years position).

Could someone advise me on the importance of age for selecting a PhD supervisor in context of a long-term future academic career?

The time for PhD would be 4-5 years, and PostDoc/Habilitation/AssociateProf usually again 4-6 years, with goal of potentially becoming Full Professor in Finance/Statistics.
First: a few people commented that thinking about your postdoc/Habilitation before even starting your Ph.D. is premature. I disagree. I have seen too many people coast along during their Ph.D. time without ever knowing what they are going to do afterwards, and certainly not preparing for their post-Ph.D. time, whether in academia or in industry. So I would say you demonstrate good long-term thinking. Already thinking about your academic career will help you prepare to work out a research program, network (more on this below) etc.

Second: there is no problem whatsoever with changing advisors between the Ph.D. and the postdoc period. To the contrary! If you stay at the same place for almost ten years, you will need to explain why you never moved, never checked out other places to work, other approaches to research. Many, many (most?) people will switch advisors at least once, or possibly even do postdocs in two different places.

Incidentally, this is why I think it is a good thing you are already thinking about your long-term future now, because it is never too early to start meeting people at conferences with your future in mind. You may just meet someone at your first conference who you could collaborate with or spend your postdoc time with.

So I would definitely recommend that you consider the older potential advisor. He sounds like he could introduce you to lots of people, and you will likely not need to pack up and move somewhere else during your Ph.D. period, which seems possible with the younger professor and which could somewhat mess up your personal life.

Of course, these considerations are all not the highest priority. You should definitely keep other aspects in mind in choosing where to do your Ph.D., like the kind of project you would be doing for either of the two professors, or whether the two of you "click" on a personal level, or what financing there is, or lots of other things you should discuss with your potential advisor ahead of time.

Finally, there is no Habilitation in Germany any more. Nowadays, Germany has moved to a more American style in academic careers. You will do a Ph.D., then a postdoc, then usually a Juniorprofessur (roughly, assistant professorship - not tenured and limited to six years), then get your Ruf to a tenured position. It's quite possible to skip the Juniorprofessur, though.