'Senior' is a word that is also used and abused in a wide variety of ways, most of which do not refer to the last year of high school.
I took a Human Growth and Development course while working towards a degree in nursing a few years ago and learned that Middle Adulthood goes through age 65. At 66 we enter Older Adulthood. I am truly Old.
Learned not long ago that CMS defines those of us who are Medicare-eligible but still working and with employer-provided health plans as --> Working Aged.
Making me working aged until the end of the year when I retire and become merely Aged.