Our state standards offer testing from age 4 (pre-K and K) but require EG/PG IQ scores to qualify for the rather modest gifted services available to that age group.  Beginning age 6, access to a small mix of different gifted services programs are available depending on a combination of ability scores, achievement scores, and teacher/parent input.  My DD's school offers a weekly enrichment pull-out for students with high ability scores but normal achievement scores.  They offer a daily pull-out for advanced academic work in math, and another in language arts, for those who meet the achievement and ability marks in those areas.
At a minimum, each student qualified for gifted services must be reevaluated through IEP review and a consultation with the teachers every three years.  If this process yields concerns that the child may not be properly placed, a full re-screening is triggered. Re-screening can also be done on request.