Quick Google shows most authoritative sources indicate 45-ish to be the
start of middle age.
Chopping 90 into 3 parts not only ignores early adulthood (starts at around 18-ish) but also assumes that the stages of life are of more or less of equal length - they aren't.
Some interesting survey stat's (particularly the variance in perception of middle age by age group surveyed - the younger you are the earlier you think middle age starts). Most popular age for being middle aged is 55:
https://d25d2506sfb94s.cloudfront.n...35n689y/InternalResults_MiddleAge_Feb18_w.pdf
As
@PaulSB indicates GP's have a different frame of reference.
My GP considers me, at 62 going on 63, to be relatively young with old age being somewhere in the late 70's upwards.