I agree with Gilles to say the place of the adjective in French is a delicate issue with few general rules.
With “ancien” you have first to see what noun it qualifies. I have to think very hard to imagine cases when, if ever, I'd use “mon professeur ancien”. First never to mean “my elderly teacher”.
We would not use “ancien” to say a person is old.
Un meuble ancien: an old piece of furniture (or an antique).
Une histoire ancienne: an old story.
Une méthode ancienne: an old-fashioned method.
According to context and stretching ideas very far I suppose in “mon professeur ancien” – if ever used – “ancien” could qualify:
- method of teaching (derogatory)
- appearance (derogatory from speaker's point of view, “ancien” here not referring to age)
- the number of years he/she's been teaching in that particular institution we're talking about.
I suppose other uses could be imagined.
In “ma jolie femme”, “jolie” refers to aesthetic canon.
But if I say “ma femme jolie”, I don't refer to a certain idea I have of beauty, but I want to say my wife is a sweet or likeable person.