From my own experience (66 years old) aging is not linear. My 20s & 30s were very much the same, apart from the wisdom that comes over time. Suddenly, right at 40, I noticed my first gray hair coming in, and I could no longer read close-up. Got my first bifocals. Also started using Viagra, more as an insurance med than for basic ability to perform.
Cruising through my 40s & 50s, I didn't have a lot of changes. I recall at 60, marveling at my balance, hearing and ability to climb the 100 steps up to the top of the San Diego convention center, without even becoming winded. I felt about the same at 60 as I did at 40. Suddenly, in my early 60s, things started going South. I had a retinal tear, & now have persistent floaters. I used to enjoy a pub lunch with a couple of pints once a week, but suddenly found I was zombified for a couple of hours afterwards and quit this simple pleasure.
Muscles became noticeably weaker from 60-65, & my a$$ seemed to flatten. Pants fall down now without cinching my belt higher up over my hips. Now I know why old men hike the waistline of their pants up like they do. I also have a paunch now, though my weight is the same. Sense of smell is greatly diminished. This seemed to happen overnight (pre-COVID). I used to wax my car twice a year, and this hasn't happened since 2018.
I retired at 62, as I couldn't stand the 12 hour days on my feet anymore. Life has become very routine. I still go out to lunch every day, but have started napping for 30 minutes when I get home. This may be my cat's fault! I've got money, but I really don't feel like travelling, like I used to dream of when younger & poorer.
I'm very thankful my health is still remarkably good. Had my first episode of angina in January... What fresh hell is this? Oh well; my dad got cancer at 67, & he never smoked or drank much at all. They say if you reach your senior years, it's either cancer or heart disease that takes you out. Given a choice, I'll choose heart disease. A stent or bypass surgery should give me another decade if I don't crump suddenly with a massive MI.
Hope this isn't too depressing. I've had a great life, and strangely am no longer frightened of dying. I fear only chronic illness & disease, which I still have not seen.
Bottom line... No, aging is not linear. It goes in spurts, much like the growth spurt of adolescents & puberty. You're just cruising along, and all of a sudden, you wake up one day and you're old!