the term "middle aged" changes as the life expectancy changes.

life expectancy is the age that humans statistically live for...

example back when life expectancy was that human beings died in their 30's middle aged was between 12-15 years old. when the life expectancy went up to statistically human beings lived to be in the age bracket of the 50 yr-60 year range middle aged was set at 20-30 years old.

now that life expectancy is in the ages of 90-120 yrs old, middle aged is now considered to be 45-65 years.

my point it may be that this friend that called you middle aged may have relatives in an older age bracket where middle aged used to be in the 20's aged bracket.

maybe you can explain to your friend what middle aged is and how it changes over time due to the fact that human beings are living longer.