It's pretty close to halfway between 0 and the expected lifespan, but that's not the common usage of "middle-aged." I always think of mid to late 40s.
I think you'll change that notion once you reach 40.
Isn't middle-aged always defined as "substantially older than me"?
Where are you living to be expecting to die at 68? The median global life expectancy is 73.5 years. I think you need to rethink how long you may live for so you can have a happy retirement.