Astrology when directing by triplicity to find each of the three stages of a native's life do I use the position of the ascendant or the relevant luminary i.e Sun in a diurnal chart?

Sunlight gives you Vitamin Ds. It you have too less vitamin Ds, you can be a hunchback for the rest of your life in lack of bone strength.

The Sun is in the main sequence stage.

Your Sun? Yeah, I can't really claim to own the sun, but Sol is in it's main sequence.

Google this.

it is in its dormant stage. the next stage is red giant.