How about turning 16 and being able to get a driver's license, or becoming 18 and registering to vote, or hitting the magic age of 21? Then, there is 30 and 40 - depending on where you are right now on life's journey. At some point there is 55 and being able to receive discounts at certain places. Retirement age used to be 65, but now it is 66 or later. The last age to receive any special considerations is at 75 when the TSA lets you keep on your shoes and jacket when you check in and go through security at the airport.
The point is that life has a series of age and other milestones (graduation, retirement, marriage, birth of a child) that we look forward to achieving or look back on as a pleasant memory from the past. Aging-in-place is a little like this.
Traditionally, aging-in-place - while a relatively new concept in terms of being defined and having a name applied to it but existing long before it was given a name - is thought of as applying to seniors. Even that term seniors is an elusive one, depending on who is making the reference and defining the term.
Seniors can mean anyone from age 50 and up or just collectively the "older" population. For the AARP, senior is 50. For many retail discounts senior is 55. For some discounts such as getting a haircut senior is 60. For social security, senior can be 62, 65, 66, or more. There are many other age thresholds used for various programs as well.
With all that said about the various age at which a person might be considered a senior and the fact that aging-in-place solutions generally have been geared toward the senior or older populations, there is absolutely no reason it has to be a senior thing. Aging-in-place can apply to anyone who wants to live successfully in their home long-term.
Maybe seniors are more likely to have selected and now occupy the home that they consider the one they want to stay as compared to those who are younger, but there is no reason younger people shouldn't want to have their homes be as safe, comfortable, convenient, and usable (accessible) as possible and be desirable to live in for years. Some may have found their "forever home" earlier in life also.
Aging-in-place is not age driven nor limited to the senior population. It is more of a strategy for addressing the quality of life in a home than being a measure of where someone is in life.