Old　tea　leaves　(OTL),　which　are　normally　discarded　as　agricultural　waste,　are　potential　sources　of　tea　phenolic　compounds.　However,　little　details　are　known　about　their　phenolic　composition.　In　this　study,　the　phenolic　profiles　of　OTL　and　young　tea　leaves　(YTL)　were　compared　by　using　ultra-high　performance　liquid　chromatography　coupled　to　hybrid　quadrupole　Orbitrap　mass　spectrometry　(UHPLC-Q-Orbitrap-MS)　analyses,　with　an　untargeted　approach.　A　total　of　60　phenolic　compounds　were　tentatively　identified.　Principal　component　analysis　illustrated　distinct　differences　in　overall　phenolic　profiles　between　OTL　and　YTL.　In　particular,　after　maturation,　flavanols　and　phenolic　acids　decreased　1.7-　and　3.0-fold,　respectively,　whereas　flavonols　increased　1.5-fold.　In　addition,　degalloylation　commonly　occurred　with　leaf　maturation,　as　evidenced　by　a　decrease　in　galloylated　catechins　and　flavanol　dimers　upon　aging.　Furthermore,　by　applying　linear　discriminant　analysis　effect　size　(LEfSe)　algorithm,　14　representative　phenolic　compounds　were　identified　which　underwent　extensive　quantitative　changes　upon　leaf　aging,　including:　rutin,　epigallocatechin,　epicatechin,　epigallocatechin　gallate,　epicatechin　gallate,　and　epiafzelechin　gallate.　In　conclusion,　this　study　provides　detailed　insights　in　the　phenolic　composition　of　old　and　young　tea　leaves,　facilitating　the　future　utilisation　of　OTL　as　a　new　and　cost-effective　source　of　tea　phenolics.