At what age does the left ventricle become bigger than the right ventricle?

The heart is one of the first organs to develop in the embryonic stage. The tissue that later becomes the heart forms as the mesoderm around 18 to 19 days of age. The mesoderm then separates to form the cardiogenic cords. The cardiogenic cords quickly develop and form the endocardial tubes. The endocardial tubes then merge to create the primitive heart tube. The primitive heart tube then quickly separates to five very distinct heart regions. One of the tubes, the bulbus cordis, develops into the right ventricle. The primitive ventricle then becomes the left ventricle. This occurs at approximately 23 to 28 days gestation. 

Although the ventricles may be the same in size, they are not the same in structure. The left ventricle tends to have a thick muscle wall and creates high pressure into the blood stream. The right ventricle has a less thick muscle wall and creates less pressure into the blood stream. 

Hypoplastic left heart syndrome (HLHS) is a congenital heart disease that occurs when the left ventricle is underdeveloped.