The generally accepted definition of middle age is 45 years old; and young adulthood ends at 40, which gives you five years to adjust I guess!
I refer to myself as middle aged when talking to people (e.g., "I'm middle aged but single"). I have even mentioned getting getting screened for cancer now that I'm middle aged to my doctor and he didn't disagree or say anything.
Are you sure middle age starts at 45?
"Middle" is just the etymology, the intended meaning is "somewhere between young man 18 to 40-something and old person (60-65+)" (childhood being its own thing).
middle age, noun: the period after early adulthood and before old age, about 45 to 65.
middle age, noun: the period after early adulthood and before old age, about 45 to 65.