Kalydeco has also been approved to use in CF patients at least 18 years old and more who have an R117H mutation.
Vertex had earlier secured approval to use Kalydeco in people with CF ages 6 and older who have one of nine gating mutations.
CF is caused by a defective or missing cystic fibrosis transmembrane conductance regulator (CFTR) protein caused by mutations in the CFTR gene.
Kalydeco intends to help the CFTR protein function more normally once it reaches the cell surface, to help hydrate and clear mucus from the airways.
In a 24-week open-label Phase 3 trial, Vertex evaluated the safety and pharmacokinetics of weight-based dosing of Kalydeco in children ages 2 to 5.
The company created a weight-based oral granule formulation of Kalydeco that is mixed in soft foods or liquids. It is available in two strengths, 50 mg and 75 mg.
The approval also includes an extension of weight-based dosing of Kalydeco to children ages 6 to 11 who weigh less than 25 kg, using the new oral granule formulation.
Vertex Pharmaceuticals executive vice president and chief medical officer Jeffrey Chodakewitz said: "Expanding the use of ivacaftor will allow younger children with cystic fibrosis to benefit from earlier treatment of the underlying cause of their disease.
"While people with an R117H mutation can exhibit a wide range of severity in their CF, once their disease begins to progress, lung function decline can be severe.
"This approval is an important advance for adults with an R117H mutation who will now have a medicine to treat the underlying cause of their disease for the first time."