  Aging in kinetics of three different phase transitions, viz., magnetic,
binary solid and single component fluid, have been studied via Monte Carlo and
molecular dynamics simulations in three space dimensions with the objective of
identifying the effects of order-parameter conservation and hydrodynamics. We
observe that the relevant autocorrelations exhibit power-law decay in
ferromagnet and binary solid but with different exponents. At early time fluid
autocorrelation function nicely follows that of binary solid, the order
parameter being conserved for both of them, as opposed to a ferromagnet. At
late time the fluid data crosses over to an exponential decay which we identify
as a hydrodynamic effect and provide analytical justification for this
behavior.
