This period runs seven years of the 120 that the Vimshottari cycle divides among the nine grahas, and a life enters it at birth if the Moon stood in one of its lord's three nakshatras, Ashwini, Magha and Mula.
The order of the nine periods is fixed and does not vary: Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn, Mercury, and then Ketu again. It is the same order in which the nine grahas own the twenty-seven nakshatras, running three times through the list, which is what ties the dasha sequence to the Moon's position at birth. The nakshatra the Moon occupied gives the period a life begins in, and the fraction of that nakshatra already traversed gives how much of the period has already elapsed.
Each major period is subdivided in the same order and in the same proportions. A mahadasha is divided into nine antardashas, each running for the same fraction of it that its lord's own period bears to the whole 120 years, and the subdivision can be carried further into pratyantardashas and beyond. That is how the system produces timing fine enough to be tested: a combination in a chart is expected to show during the period and subperiod of the planets that form it.
Ketu opens the list as the tradition states it, which is a convention of presentation rather than a claim that any life begins there. A life begins in whichever period the Moon's nakshatra assigns, and all nine are equally likely at birth in proportion to their lengths.