Short answer
HDB classifies towns as mature or non-mature. Mature estates such as Tampines, Toa Payoh and Bishan are the older towns, where less land is left for new public housing; non-mature estates include Punggol, Sengkang and Woodlands. New BTO flats are no longer labelled this way: since the October 2024 BTO launch they are classed Standard, Plus or Prime.
Mature estates
Ang Mo KioBedokBishanBukit MerahBukit TimahCentral AreaClementiGeylangKallang/WhampoaMarine ParadePasir RisQueenstownSerangoonTampinesToa Payoh
Non-mature estates
Bukit BatokBukit PanjangChoa Chu KangHougangJurong EastJurong WestPunggolSembawangSengkangWoodlandsYishunTengah
Tengah, the newest town, is listed with the non-mature estates by outlets such as EdgeProp and 99.co. The Standard, Plus and Prime labels apply to new BTO flats from the October 2024 launch; existing flats and earlier launches are not affected. Check HDB for the current list. The classification affected rules such as the ethnic integration policy and some BTO conditions.