So far I have seen three type of people in my life.
1- Able to switch instantly
2- Taking some time to switch
3- Deceivers

In my personal opinion, first two kinds have its pros and cons. Feelings and emotions are the very things which makes us human but sometimes these things also become our weakness due to our expectations from others.

Those who are able to switch instantly can be regarded as Cold-bloodied humans because they won't feel anything at all. It might seem perfect at first but their life would be more of mechanical only.

Those who take some time to switch and process their emotions, we have large portion of them and it is totally fine to continuously replay thoughts in your mind because without all of them - we have nothing. It got nothing to do with age but what we can do is control our REACTIONS to emotions and improvise them with TIME.

BUT I really despise the third kind who claims to take some time to process emotions but their actions will say otherwise by depicting their reactions instantly to emotions in a complete switch-over manner.