i am currently a non-pediatric setting OTR but my 2+years experience with pediatric OT from my previous job may validate my 2cents.
from how i can recall, in terms of age, laterality begins:
at 2-3 years old where it is somewhat deceiving since a child may look ambidextrous for the most part and is termed "preference"
at 5 years old, it's the child's "dominant" side - now, one may distinguish frequent use of one side, apparent during reaching activities/test;
by the age of 7, it is considered concrete and thus, termed "handedness." Moreover, handedness may have something to do with the brain dominance (whether left or right hemisphere-dominant)...so if that is established, that it - that's the "strong" side that influences your intellectual capacity as well.
points to ponder:
if a child has no known medical condition/s affecting physical (motor), mental (cognitive, sensori-perceptual, cognitive d/o) and/or combination of both then, i could say that the parameters mentioned above can be used as the "normal" parameters to validate the appropriate intervention.
*** to answer you question using these parameters, normal children who belong to groups of Kinder to 1st grade would be in the age-range of 5 to 7 y/o (i might be wrong), you may be able to strengthen the side that the child practically uses often to improve and establish your plan of intervention.
if there is an identified condition, the "age" parameters should be viewed in correlation to the developmental maturity (mental-cognitive) in synchrony with the parameters of growth (physical). you might want to contemplate on qualifying your evaluation not by just "age" per-se but by assessing chronological age versus mental age.
there's a good reference about growth and development that discuses the evolution of handedness...but i failed to recall the title nor the author.
i hope this helps.