Production could double year 6 compared to 4. Not on my trees closely spaced where production can peak as early as year 4.

Most of what you read about production vs age will be based on years after planting. A big tree with big roots properly planted and grown could have a one year, two years max, advantage over a small tree. But growing conditions and training after planting are generally more important than age or size at planting.

typically the bare root trees I buy are third leaf. nurseries pot bare roots and charge more for them, not really any advantage there unless they are no longer dormant.

When discussing tree production it is important to know such things as root stock, variety, soil and climate and other environmental conditions, as well as pruning habits etc. It might be a good idea to contact your local extension office for a better idea of what to expect.