Ykhjuy

I am starting a new postdoc position with a professor who's retiring soon (<5 yrs, judging from the grad intake).
I am interested in pursuing academic job and it worries me that I would not be able to count on him during my career for support after he retires.
Am I overly worried and exaggerating tha benefit of having a senior scientist interested in your work in the early stage of your academic career?
Right now, I am thinking of doing as much work as I can and using his connections to do a second postdoc with younger and active PI.
Any advice would be appreciated!

You are overthinking it and maybe trying to plan too far in advance. Your current prof, after retirement, will most likely still be able to help you. He will still have his contacts and may still give you some, if less, feedback on your ideas.

You are right to maximize what you have now. But as time passes you will have the opportunity to fine-tune your future. You will know more in a year or so. You can explore with your current prof what he intends after retirement. It may be retirement in name only, perhaps. You can also get connected to his network as you work along, so that his direct presence is less important.

Far worse if someone leaves suddenly for any reason. That can leave you stranded. This situation seems well controlled with a lot of planning time.

Before worrying too much, I think you should simply ask the professor about his plans for the future. Is he actually thinking about retiring, or is this a temporary wind-down? Also keep in mind that some people want to retire from teaching and administrative duties, but stay active in research or writing for a lot longer, while others will simply vanish the day of retirement. (The latter is rare, of course, but still happens.) The point is, if you form a good relationship with this professor, and they aren't the vanishing type, you might very well have their support beyond retirement.

Some advantages of working with someone late in their career: They will be well-networked and knowledgeable in your field, so a good source of advice and expertise. As @Anyon states, many retired researchers continue to publish and attend conferences, so ongoing collaboration is possible. A good person of this type to work with would have many well-placed former students/postdocs/junior colleagues that he or she still works with, and be receiving recognition (awards etc) for their career contributions to your field. You would want to avoid someone who has maintained feuds with their peers since they will be prominent in your field too.

Some advantages of working with someone early in their career: They are looking to build up funding or reputation too, so there will be opportunities to collaborate on future projects. A good person of this type to work with would be getting early-career recognition (asked to chair sessions at conferences, for example) and have a good funding track record. You would also want to make sure they have a personality for collaboration. Some people can get a good start based on their own talent but are not able to mentor junior colleagues well.

Best of all is to get a mix of both, as you suggested.