children automatically have their own sense of age/sizing. example how do you know this little is three, she had to have some sort of concept of age in order to tell someone inside or out how old she was. I have a friend who has a three year old. ask them how old they are they hold up their fingers. when I ask who is older me or them they point to me, why because Im bigger, why because I have wrinkles, why because ... three year olds have their own concepts of aging.

the same with alters. DID type alters are a very special kind of alter, they have their own concept of who and what they are, their own job, purpose, reason for being. this distinction carries through with things like the alters concept of self, how they interact, what they wear, how they behave, they even have their own concept of sense of aging, the DSM 5 explains alters this way.....

Dissociative Identity Disorder 300.14 (F44.81)

A. Disruption of identity characterized by two or more distinct personality states which may be described in some cultures as an experience of possession. The disruption in identity involves marked discontinuity in sense of self and sense of agency, accompanied by related alterations in affect, behavior, consciousness, memory, perception, cognition, and or sensory-motor functioning. These signs and symptoms may be observed by others or reported by the individual.

B. Recurrent gaps in the recall of every day events, important personal information, and or traumatic events that are inconstant with ordinary forgetting.

C the symptoms cause clinical significant distress or impairment in social, occupational or other important areas of functioning.

D. The disturbance is not a normal part of a broadly accepted cultural or religious practice.

Note in children the symptoms are not better explained by imaginary playmates or other fantasy play.

E. The Symptoms are not attributable to the physiological effects of a substance (e.g. Blackouts or chaotic behavior during alcohol intoxication) or another medical condition (e.g. complex partial seizures)

along with this diagnostic criteria are 5 pages that go into more detail in categories of diagnostic features that explains the above diagnostics in more detail, Associated features supporting diagnosis, the statistical prevalence of the disorder, the development and course of the disorder, Risk and Prognosis Factors, Culture Related Diagnostic Issues, Suicide Risks and the Functional consequences of this disorder. The DSM 5 also addresses Differential Diagnosis (other dissociative disorders that share the same symptoms) and Comorbidity (having more than one diagnosis)
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in short what criterion A means is that there are sure fire ways alters distinguish their self from the person in which they reside with in. even though your alter is only three they will be able to discribe or draw the differences between them and the person they reside with in.

example my non verbal three year old when asked what I look like drew her perception of me and then when asked what the three year old looks like, the alter drew a very different picture of how she perceived herself to be. then when the therapist asked the alter look at the hands, who's hands do you see, she pointed to the picture of me. when asked how does she know the alter drew pictures of hands hers were smaller than mine.

my suggestion give your therapists idea a try it may surprise you to discover how much your alter really knows about the differences between you and the alter.