Age-Related Variation in Flavonoid Composition, Human Pancreatic Lipase Inhibition, and Preliminary Brine Shrimp Lethality of Rhododendron adamsii Leaves and Sagan Dali Tea

Rhododendron adamsii Rehder is an important Asian ericaceous species widely used throughout the region as a food plant for producing sagan dali tea. Although the leaves of R. adamsii are consumed at different developmental stages, potential variations in their metabolite composition and bioactivity have not yet been scientifically investigated. To address this knowledge gap, first-year, second-year, and litterfall leaves of R. adamsii, together with sagan dali tea decoctions, were subjected to LC–MS profiling and quantification, in vitro assays measuring human pancreatic lipase (hPL) inhibition, and toxicity evaluation using the brine shrimp lethality (BSL) assay. The results showed that leaf age significantly affects (p < 0.05) phenolic diversity, total content, and antilipase activity in sagan dali tea, establishing the following performance ranking: first-year > second-year > litterfall. Leaf senescence caused a decrease or disappearance of flavonoid glycosides, accompanied by an increase in free aglycones, ultimately altering the tea’s chemical composition. Specifically, the predominant compounds in first-year leaves were myricetin 3′-O-xylopyranoside (881–917 mg/100 g) and quercetin 3-O-galactoside (672–714 mg/100 g), whereas free myricetin (427–451 mg/100 g) predominated in the litterfall. The highest antilipase potency of sagan dali tea was recorded in first-year leaves (IC50 30.65–33.47 μg/mL vs. orlistat 35.28 μg/mL). This activity was associated with the presence of myricetin, quercetin, and gossypetin in both free and glycosylated forms, with IC50 values lower than that of orlistat under the tested conditions. Furthermore, the BSL assay showed LC50 values > 1000 μg/mL, indicating low lethality of sagan dali tea to brine shrimp across all categories under the tested conditions. However, these results provide only preliminary information and do not establish food or human safety. In summary, the age of R. adamsii leaves is a critical parameter determining phytocomponent concentrations and associated bioactivity while consistently maintaining low brine shrimp lethality under the tested conditions. These findings provide a biochemical basis for further research into the optimal use of R. adamsii leaf material.