I don't think that the adjustment has to do with age, more individual personalities - if that makes sense? Like some people just adapt better to British humour than others, as an example.
As a personal example, I know I totally prefer the customer service (barring food industry) in England to that in the US. I liked moving here and being left alone in shops as opposed to having somebody so over the top friendly but I know that loads of people (even Brits) prefer that and feel customer service here leaves a lot to be desired. I also know I wasn't the type of person to complain about service standards unless it was REALLY bad but I knew plenty of people in the US who were more than comfortable voicing their concerns and unhappiness with a service they're paying for. For me, my personality made it easier to adapt to the way the culture here is in that area but there are others who were younger as well that found that very frustrating because it didn't match their personality.
British people loooovvvveeee to moan, so they will definitely complain..just not directly to the people they need to be complaining to (as you mentioned).
Sorry to ramble, I'm trying to distract myself from catching up on work but my brain is just a massive jumble at the moment. Hope that all made sense lol