There are a couple of parts to this problem.
Expressing age
There are very few concepts in the International Edition that express a condition happening relative to the patient’s age.
For example 154051000119101 |Family history of malignant neoplasm of colon over age 50 (situation)|.
The only concept I can find that referred to age in the attributes is 713068007 |Over 75 years of age health check declined (situation)| which uses an Associated procedure (attribute) of 134186004 |Over 75 health check (procedure)|.
One of the reasons that not many of these concepts exist is that the OWL profile used for reasoning can not classify these age related concepts in the way we expect. It will not for example classify “Over 75 health check” as a subtype of “Over 70 health check”, even though logically age 75 is “over 70”.
Which hierarchy
The concept 445518008 |Age at onset of clinical finding (observable entity)|, like other observable entities, is intended to be used to express a question. The answer to that question would be a separate concept or data point.
For example we could have a field where the question is bound to the concept 773251000 |Age at onset of menopause (observable entity)|. The value of that field could be number input.
Not known
SNOMED CT is for expressing what is known. Concepts relating to “not known” or “other” have been intensionally removed. There are a few execeptions like “169963001 |Birth details not known (finding)|” and others which are left over from 20 years ago.
In general we advice just stating what is known.
Answer
I think the precoordinated concept that is closest to what you want to express is 444862003 |Childhood obesity (disorder)|. This uses an 246454002 |Occurrence (attribute)| of 255398004 |Childhood (qualifier value)|. However this covers children up to the age of 18. For less than five years the concept for “Early childhood” is closer.
If only using existing concepts I would suggest this postcoordinated expression:
=== 414916001 |Obesity (disorder)| :
{ 246454002 |Occurrence (attribute)| = 713152004 |Early childhood (qualifier value)| }
{ 363714003 |Interprets (attribute)| = 363808001 |Measured body weight (observable entity)|,
363713009 |Has interpretation (attribute)| = 281302008 |Above reference range (qualifier value)| }
This expression is a refinement of the Childhood obesity (disorder) concept, using Early childhood (qualifier value) for the Occurrence.
Chapter 5.3 of the SNOMED CT Practical Guide to Postcoordination introduces Transformation Level 1 which is designed to simplify the process of authoring and maintaining postcoordinated expressions. When using a terminology server that supports Level 1 the above expression can be simplified to:
=== 414916001 |Obesity (disorder)|:
{ 246454002 |Occurrence (attribute)| = 713152004 |Early childhood (qualifier value)| }
If you wanted to be even more explicit and say that onset was before 5 years then a new precoordinated concept would need to be created to express that age range.
It’s okay that the “not known” aspect is not captured in this expression. This expression already has those semantics. We have not given a specific age, only an age range. This is the same as saying some time during early childhood.