Unlike hydrodynamic developed flow, a constant profile shape is used to define thermally fully developed flow because temperature continually approaches ambient temperature.  Dimensionless analysis of change in profile shape defines when a flow is thermally fully developed.
Requirement for thermally fully developed flow:
Thermally developed flow results in reduced heat transfer compared to developing flow because the difference between the surface temperature of the pipe and  the mean temperature of the flow is greater than the temperature difference between surface temperature of the pipe and the temperature of the fluid near the pipe boundary.