The average female life expectancy in Canada is around 84. Broads are sensitive enough about ageing without movies telling them they're cooked by their late 30s (or even earlier, since the film is not specific about the age at which one's ass becomes "old").
I think one understands that their life value, exuberance, spirit / whatever you want to call it is effectively over at 40.
Sure you can rail against it for a bit but psychologically you certainly have the formation of understanding that the cheque's in the post. So yeah, it's okay thematically to describe 39 as old...
Er, maybe... But I was talking more psychology how you know it's gone by the age of 39. Maybe you're fine walking until your're 70/80 - who knows - but that's another matter.