Speaking as a female Masters athlete, I would tell your athlete that it may be psychological but it is NOT an “age thing”. I have run faster 100 and 200m times as a 45yr old than I did as a 40-44 yr old. I used to die in the last 20-30 meters, but this year I changed to a long - short program and also added a LOT of open 400m to my early season meets. After that I no longer feared dying in the 200. Don’t know if this would help your runner - but it’s just my 2cents.