Fish oil supplementation is suitable for most ages, including kids older than three. For kids under the age of three, you'll want to check with your pediatrician to make sure it’s safe. The most important factor for all ages is appropriate dosing. Children will require much less omega-3s than adults. Therefore, it's best to purchase a fish oil specifically made with a lower dose for kids. Other sensitive populations include people taking blood thinners, as fish oil has a slight blood-thinning effect. In any case, it's important to check with your healthcare provider before starting a new supplement such as fish oil.