Can a retroactive support claim be brought years after the child has already turned 18?

Potentially, yes, though it becomes a more difficult claim to bring the longer the delay stretches past the child's eighteenth birthday, since courts weigh the same factors that apply to any retroactive claim, including the reason for the delay, the payor's conduct, the child's circumstances during the relevant period, and the hardship a large award might now cause. Turning 18 does not automatically extinguish a support obligation, since a child can remain entitled to support past the age of majority in certain circumstances, such as continuing full-time education, and a retroactive claim can in principle address a period when the child was still entitled to support even if that entitlement has since ended.

Courts are generally more cautious the further removed a claim is from the period the support was actually needed, since evidence about the child's actual needs and the payor's income during that time becomes harder to reconstruct with the passage of time. A claim brought promptly after learning of a payor's concealed or increased income is viewed very differently from one brought many years later with no clear explanation for the delay. Because this is a fact-heavy, time-sensitive area, anyone considering a claim like this should get legal advice promptly rather than assuming it is barred, or that it will automatically succeed.