Use of Devoir

Question

In exercise "Fishing with my father C1". Could you please explain the use of devais in je ne devais pas avoir plus de dix ans. If it's; I mustn't have been... isn't that the passe composse ?

Accepted Answer

Hi Michael, 
This is quite an idiomatic use of
devoir + avoir +age
Je devais avoir 4 ou 5 ans = I must have been 4 or 5 
Je ne devais pas avoir plus de 5 ans = I couldn't have been more than 5 
Je devais avoir une vingtaine d'année quand ça s'est passé = I must have been about 20 when this happened
And no, you couldn't use the passé composé as it indicates a period of time.

Answer

Same confusion over use of 'devais' - not least because the phrase to be translated is 'I couldn't have been more than 10'.  Is «Je n'aurais pas pu avoir plus de dix ans» acceptable but just not the way it is said?