To repeat - there is no way of knowing the collapse time based upon observation of the Sun itself.
That information is lost in the collapse process. No present day observation of the Sun can give you that information.
What part of this can you not fathom.
No observation of any quantity maintains information of that time. The star in the collapse passes through a fully convective phase. As such it is homogeneous and rotates as a solid body. Thus no differentiation of the internal structure occurs so no light elemental abundances we can measure now allow us to probe this time period during the stars formation. Since it rotates as a solid body (due to convection) we cannot extrapolate backwards in time. We don't know the initial Sun's total angular momentum.
Thus when referring to a star's age the zero point is usually taken as the time it arrives on the main sequence (or first forms a radiative core - these are close to the same time.)
Now from that point on we can set a clock so to speak.
But the collapse itself HAS NO EFFECT on the subsequent evolution.
The estimates for a collapse time are based upon basic physics plus observations of young stellar objects (in various stages) in star forming regions. You can then estimate the collapse time.
BUT TO REPEAT, the collapse time is not something we can gather from observations of the Sun taken at the present day.