It's from end of tournament
Nope , beginning.
Here is the rule(bolded part)
A. AGE ELIGIBILITY RULE
Player Development requirements and the Age Eligibility Rule (“AER”)
are applicable to competition in Professional Tennis Tournaments
worldwide, including all WTA Tournaments and ITF Women’s Circuit
events that receive ranking points.
a. Competition in a Professional Tennis Tournament
For the purposes of this rule, competition in a Professional Tennis
Tournament is defined as participation in the singles and/or doubles
Main Draw, or the singles and/or doubles Qualifying Draw of
a WTA Tournament or ITF Women’s Circuit event. Each
Tournament in which a player earns ranking points (singles and/or
doubles) counts towards a player’s allotment of Tournaments
under the AER. However, participation in the draws of the ITF
Women’s Circuit events in which a player does not earn ranking
points does not constitute competition in a Professional Tennis
Tournament under the AER.
For the purposes of this rule, a player’s age is determined by her
age as of the date of start of a Tournament’s singles Main Draw.
A player 18 years old or older no longer is subject to Part A of this
rule but is required to continue participating in Player Development
(Part B) until she receives notification of graduation from a WTA