I'm 40 and just enlisted into Inf Aussie reserves, 25 press ups,55 sit ups and 2.4km in 12:41 are whats required if you come in as a renlist. Otherwise its 7.6 on multi stage fittnes, 15 press ups and 45 sit ups. Yes there were 19 year olds who failed.
An ex copper sat next to me was 55 and wanted to enlist and do basic training, provided you are are fit and capable you are given the chance.
Bewarned it takes about 8 months.