Clausena lansium (Lour.) Skeels, commonly known as "wampee," is an excellent food ingredient of medicinal value. Effects of leaf developmental stages on the composition of phenolics, flavonoids, and antioxidant activity were investigated. Phenolics composition was studied using HPLC-PAD, whereas antioxidant activity was estimated by oxygen radical absorbance capacity (ORAC) and cellular antioxidant activity (CAA) assays. Increase in bound flavonoids, quercetin, and cellular antioxidant activity was observed in bound and free fractions at different stages of leaf development. Predominantly, quercetin and ferulic acid contents were high in free and bound fractions of old leaves. In addition, phenolic components depicted highly significant positive association (p < 0.05) with antioxidant activity. Overall, old leaves of wampee have utility value similar to leaf buds, so they could be a more sustainable and economical source of bioactive compounds for commercial application in nutraceutical and pharmaceutical industries.