It makes sense for the order to be older-younger, because high school has a longer day than elementary school, so pushing their start time to after the elementary school will make it so the elementary kids are out at noon or just after and the high school kids won't be done until dinner time. Symmetrical makes it easier for everyone's schedule.
When I was five, kindergarten was a half of a day. Every other grade was a full day, and I'm fairly sure kindergarten was full day by the time I graduated.
