While that is true De Gea was 12 years younger when he joined United. You usually have more patience with younger players (unless you are Barça fan, lol) but people usually also expect immediate impact from experienced players like Bravo, especially if he's replacing one of the club legends (Joe Hart) that fans love.
I believe that every player (no matter how old he is) deserves a full season to adapt to his new team before we can call him a flop, especially if he goes to a different league (when he also has to adapt off the field to different culture and language, which makes his on the field adaptation even harder) than he played in for most of his career but De Gea and Bravo cases are just different. United were willing to go through problems De Gea had in his adaptation period because they thought he can be their longterm GK (and they were right). On the other hand, even if Bravo somehow finds himself again I doubt he'll stay in City for more than 2 or 3 years (he'll most likely leave when Pep will if not before that).