Dear OP,in the first sentence of your query you asked for a stage in life as opposed to specific age.
re stage in life:
I do think men and women somewhere bet40-45 start to be viewed by themselves and others as getting older.I think a reality starts to sink in more between 45-55 as a result of life experiences.(A stark example at this stage is having to take more care of your own parents, or to a lesser extent a doctor reminding you that you cannot eat all the things you used to enjoy).
Re being a senior: Once you are zoche to be healthy and past 70, it is all a matter of Mazel, and Davening to Hashem to sustain your mental and physical Koach as long as possible. Staying active, a caring family,positive mindset and remembering that age is just a number(just because you are numerically a senior doesn’t mean you always have to act like one) are all key. You can have a 85 year old doing better then a 70 year old if they are able to stay active and maintain positive outlooks on their station in life.
Re scary: Aside from the fear of infirmities that can come with old age, each new stage of life while many times happy, is scary for anyone concerned about accomplishing what they were supposed to. Whether it be while in yeshiva, college, career, or relationships… there are certain things you set to accomplish in all stages and areas of life.
While Boruch Hashem, we know Hashem judges favorably and with Rachmonus no matter what, people tend to be harsher on themselves for what they could have or should have done.The fear of time passing and rectifying faults REAL OR PERCEIVED is inevitable for any senior. As a senior you have a magnified fear of perhaps not using your G-d given tools to their greatest extent and worst of all, mistakes made or regrets over things that you could have done differently. It is important to remind yourself at these times what Boruch Hashem you did accomplish.
May Hashem guide us through all our stages of life, that we should be able to accomplish and do what is right and be a person Sameach Bichelko.