Final answer:
The developmental stage marked by the center of mass being in the abdomen, accompanied by reaching and bilateral symmetrical control, is found in infants around 3 - 4 months old.
Step-by-step explanation:
The time frame of a child's life described by the center of mass being located in the abdomen, reaching, and bilateral symmetrical control is typically associated with infants at the age of 3 - 4 months. At this stage, an infant develops the ability to roll from front to side, can lift their head 90 degrees while lying prone, and sits up with support, showing that they are beginning to control their body in a symmetrically bilateral manner. This is also the period when they begin to hold their head steady for brief intervals, which indicates the beginning of more controlled motor function and balance.