You should apply for Child Benefit as soon as possible after your child’s birth. HMRC will usually only backdate a new claim by up to three months, so delaying may mean losing payments for earlier weeks. You can claim online or by completing form CH2, and you may still be able to claim for an older child if they meet the age and education or training rules. Check the latest guidance and eligibility details on GOV.UK, as HMRC decides the claim based on your circumstances.