GWRs age as an average will improve in the near future when the 175s arrive and the HSTs leave since they are younger than the 15x.
I calculated it other way, and goes up
lose 20-24 vehicles (incl spares) around 45 years old
gain 70 vehicles about 24 years old
changes the current 1047 vehicles at avg 15.0 years old
to about 15.5 years (exact age & quantity of HSTs slightly affects exact figure) because increase fleet size with vehicles 9 years older than average
Of course if then get to 2028 before any other changes (and DfT is currently showing replacement of existing diesels by 480 vehicles 2029-2031), then by 2028 average age will be about 20 years