Hi McG
I do not believe that he lost his legs in WW1. The NAA record shows that he was not in France at all and was returned to Australia and discharged through ill-health - gastric ulcer. He seems to have been a bit free and easy with the truth on occasions and it would not surprise me if he made up his age for the enlistment.
It seems there are at least two men with the same first and surnames - the Frederick John YEOWART b 1886 that Merlin has found died Jan-Feb-Mar 1973, aged 86
Death Registered Birkenhead; Vol 10a, p223
The Fred YEOWART found all over Australia dies in WA in 1954.
The Frederick YEOWART, aged 15, shown in 1901 census at 58 Wendell St, Toxteth Park, with parents Robert and Rosina, and sister Kathrena, has a middle initial "S". However, I cannot see the Frederick John on the 1901 census

(I note that the NAA has another listing for a F YEOWART which is apparently pension files for WW1 - not digitised.)
Judith