2009 H1N1 pandemic influenza (A(H1N1)pdm09) virus infected large numbers of people worldwide. Recent studies suggest infection with A(H1N1)pdm09 virus elicited cross-reactive anti-hemagglutinin (HA) memory B cell response to conserved regions of HA. However, the breadth and magnitude of cross-reactive immunity in children and adults following A(H1N1)pdm09 infection are unknown. Methods: We investigated serum anti-HA immunity to a number of group-1 and -2 viruses in children and adults using hemagglutination inhibition (HAI), enzyme-linked immunosorbent assay and virus neutralization assay. Results: Applying hemagglutination inhibition (HAI) titers ⩾40 against A(H1N1)pdm09 as threshold of sero-positivity, we observed significantly higher levels of anti-HA antibodies to a number of virus subtypes, including those neutralizing H5N1, in subjects with HAI titer ⩾40 than those with HAI <40. Adults demonstrated broader and stronger cross-reactive anti-HA antibodies than children, including cross-reactive anti-HA1 and -HA2 antibodies. By comparison, individuals with serologic evidence of recent exposure to seasonal H1N1 or H3N2 did not show such broad cross-reactive immunity. Conclusion: Our results suggest individuals exposed to A(H1N1)pdm09 virus developed a broad and age-associated cross-reactive anti-HA immunity which may have important implications for future vaccination strategies to enable protection against a broader range of influenza viruses.