Crystallization under isothermal condition of supercooled liquid and amorphous silicene (a-silicene) models has been studied via molecular dynamics (MD) simulation with Stillinger-Weber (SW) interaction potential. Supercooled liquid and a-silicene models containing 10^4 atoms are obtained via the rapid solidification process from the melt. At each given temperature below and above Tg, models are annealed from 5 ns up to 8 ns in order to investigate aging effect on two-dimensional structural arrangement of disordered Si-atoms. Time dependence of thermodynamic and structural quantities is analyzed including potential energy, radial distribution function (RDF), coordination number, buckling degree, ring and bond-angle distribution. We find that 2D-crystallization of supercooled liquid and a-silicene exhibits a first-order behavior. Time-temperature-transformation (TTT) diagram exhibits a commonly nose-shape by analyzing a wide temperature range from supercooled liquid to amorphous state. High critical cooling rates of a-silicene are found indicating low glass-formation ability of the system. Thermodynamic properties of crystallization of silicenes attained after-aging are studied in details and we clarify a novel scenario of crystallization. A homogenous tendency of natural quenched-in nucleation atoms aggregate into larger clusters in 2D Si supercooled liquid sheet. Consequently, a polycrystalline layer of silicene is developed with decent low-numbered chain defects as grain boundary. In contrast, crystallization of a-silicene shows a partial crystallization behavior: Crystal clusters are found to exhibit a heterogeneous growth in models obtained below Tg. Crystal formed from a-silicene is a quasi-equilibrium state with a large number of defects still existed in models after a long relaxing time.