Wikipedia reports old age is life's later part, coming after middle age with facets of deterioration. A basic definition, outside the study of gerontology.
Aging effects you in different ways. How old you feel or perform changes each day. Sometimes it depends on the time of day.
I am blessed with good health for being almost 70. Sure, I could be more active and loose some weight, especially after the holidays. Yet as I go about my daily life, I see challenged, devitalized middle-agers I out do.
I don't like exercising, especially outside my home. I prefer to give myself reasons to get moving. I do photo treasure hunts. If the weather is nice, I go to Lauritizen Gardens or the Henry Doorley Zoo and take pictures. When it's better to be inside, I go to an indoor mall walking and snapping photos.
I shoot at a gun range or travel to an SCA event. While not exercise per say, it is activity. These activities also provide mental exercise. Talking with friends and those I meet provide many, sometimes new things to think about.
The way you think about your aging is as important as your physical ability. While I am blessed with health, I also have a can do attitude. Being the youngest, I grew up wanting to be like my older brothers. Then I went through the Helen Reddy stage of I Am Woman Hear Me Roar. I also participated in sports that espoused independence, such as alpine skiing and SCUBA diving. It's been years since I did those sports, but the attitude remains.
I have seen the effects desiring to be old brings. My Dad, born in Austria-Hungary wanted to be seen as the patriarch. He also lived through the institution of Social Security. While his vocation was physically challenging and debilitating, his “old age” outlook contributed to his decline.
The reality is you age. How dramatically and when seems to vary. When old age begins cannot be universally defined because it shifts according to the frame of reference.
I have little reminders. I use to be a barefoot girl, even into my 60s. Due to bunions, I now have constant annoying tingling in my feet. If I wear shoes I don't notice it. Now, I constantly wear shoes. A small adjustment. There's more, but I don't dwell on them, because they are annoyances, not suffering.
I focus on my blessings. I am thankful I have strong comfortable, hands. So much depends on them. My calligraphy and painting, hand sewing, knitting, and shooting. Even reading an electronic device or keyboarding. For me the aches seem related to the length of time I spend doing something. So I do something else or adjust my posture.
Aging is not a constant. It wasn't when you were young. The number of years that imply "old age" varies by background and time period. Thus, old age is a social concept, not a developmental stage. Where life expectancy has broadly shifted beyond 80 years the definition of old age has advanced as well. The new, later cultural definition of old age reflects back on how long those around us think you “should” be performing activities, and when you are old.