I think people are confusing "simple" dissociation with the type of dissociation that occurs in people with DID.

"Simple" dissociation can happen at any time to anyone, but this is more like highway hypnosis or daydreaming or in situations where a person begins to get very anxious, they may feeling like they are floating or get dizzy.

"Severe" dissociation where alters are created only occurs in childhood. And the symptoms of DID exist since childhood because the purpose of the alters is to protect, so when another alter appears, time is usually "lost."
Alters have been present since they were created and they behave and do their "job" all the time. They don't get created in childhood and only appear later in life. They are created to serve a purpose and when needed, they "switch" into their job, usually unknown to the host. They can be out for seconds or for weeks. When needed, they responded completely unconsciously.

It can take many years of hard work to develop communication among alters so that a person can gain some control.

The alters/parts of a person with DID vary but there are usually very similar patterns in what exist. And if you read the research, most T's will not diagnose DID unless those characteristic alters are present (that is debated)

There are no cases of DID developing in adulthood.

In fact there have been numerous studies on people severely traumatized in adulthood in a variety of situations such as abuse or hostages to understand if DID develops. It does not.

There are many reasons why children have the ability to dissociate to the extreme to develop DID that differ from adults, including things such as the developing mind, the perception of the world, the brain chemicals, and the psychological stages of development.

But, people's experiences are different and only a qualified T can diagnose a complex disorder such as DID.

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*"simple" is not used as an insult, but just a way to characterize differences