Abstract: Lu’an Guapian stands out as a unique variety of chinese green tea, traditionally processed using only single fresh leaves while rigorously excluding buds and stems. However, the growing shortage of agricultural labor has led to the widespread application of mechanized harvesting, which inevitably results in raw materials mixing leaves of different maturity stages. This significantly compromises final product consistency and stability. Therefore, understanding biochemical and sensory alterations induced by leaf maturity is essential for establishing scientific grading criteria and optimizing processing techniques. This study manufactured Lu’an Guapian samples from fresh leaves harvested at three positions relative to the bud: the first leaf (L1GT), second leaf (L2GT), and third leaf (L3GT). A comprehensive multi-disciplinary approach evaluated these samples. Computer vision systems and electronic tongue technology quantitatively characterized infusion color and taste intensity. Chemical composition was rigorously analyzed using ultraviolet-visible spectrophotometry, high-performance liquid chromatography, and liquid chromatography-mass spectrometry metabolomics. In vitro simulation assays measured antioxidant capacities via 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assays, plus hypoglycemic potential through alpha-glucosidase and alpha-amylase inhibition. The dose-over-threshold (DOT) method identified key metabolites regulating taste differences among grades. Empirical results demonstrated that leaf maturity exerts significant influence on sensory attributes, physicochemical indices, and health-promoting functions. Regarding sensory characteristics, as maturity increased from L1GT to L3GT, infusion greenness rose significantly from 6.79 to 8.66, whereas yellowness declined sharply from 54.80 to 24.40. Concurrently, electronic tongue response values showed synchronized downward trends; the bitterness (8.95), astringency (5.87), umami (19.45), and richness (11.61) of L1GT all significantly exceeded those recorded for L3GT (8.00, 4.63, 17.69, and 9.88), indicating that tender leaves possess a markedly more robust and complex flavor profile. In terms of chemical composition, the contents of tea polyphenols, free amino acids, and total flavonoids showed significant decreasing trends with increasing maturity, while soluble sugars and total chlorophyll conversely increased, reflecting differential regulation of metabolite accumulation driven by leaf development. Regarding in vitro biological activity, both antioxidant capacities and enzyme inhibitory activities displayed significant dose-dependent relationships with leaf tenderness. L1GT demonstrated superior functional performance, exhibiting the lowest median inhibition concentration (IC₅₀) values for DPPH and ABTS radical scavenging at 14.16 and 239.40 µg/mL, respectively. Similarly, L1GT manifested the most potent inhibitory effects against α-glucosidase (IC₅₀=32.72 µg/mL) and α-amylase (IC₅₀=32.24 mg/mL). In the context of metabolic mechanisms, metabolomics analysis identified 24 differential compounds. Through DOT screening, eleven core substances were pinpointed as critical contributors to taste formation. Notably, the abundances of myricetin 3-O-galactoside, kaempferol 3-O-rutinoside, kaempferol 3-O-glucoside, epigallocatechin gallate, epicatechin gallate, caffeine, L-glutamic acid, and gallic acid were significantly elevated in L1GT. Pathway analysis suggested these variations were primarily governed by regulation of the phenylpropanoid biosynthetic pathway, the tricarboxylic acid cycle, and the xanthine methylation pathway, which collectively lead to the intense bitterness, astringency, and umami characteristics of younger leaves. In conclusion, fresh leaf maturity acts as a decisive factor in determining the sensory quality and health-promoting functions of Lu’an Guapian. Young leaves are characterized by high concentrations of polyphenols, flavonoids, and amino acids, thereby combining potent antioxidant activity with a rich taste profile. Conversely, while mature leaves accumulate more sugars, they lack the high biological activity found in tender shoots. This research provides a solid theoretical basis for the scientific grading of Lu’an Guapian raw materials and lays the foundation for developing targeted processing strategies to realize the high-value utilization of fresh leaves at different maturity levels.