I have 12 Turkey eggs. They are all different ages. I have the incubator set for 100F with humidity levels that flux between 56-60%. I hand turn them three times a day. I candled them recently and I saw that some of them are ~20-24 days. I know that they should hatch around day 28 but, I am inexperienced and worry that I might not know when to stop rotating the eggs! 
Is there a tip or trick that will help me distinguish age with greater accuracy?
Is there great harm in turning them up to the day they pip?

Is there a tip or trick that will help me distinguish age with greater accuracy?
Is there great harm in turning them up to the day they pip?