I don't set eggs over 8 days old. I know a hen can set on eggs older that 8 days and get a good hatch but it seems rare for them to hatch well using and incubator. If I need to have more eggs I'll split the hatch. Set some at 7 days and 7 days later finish filling the incubator. Chicks a week apart integrate well for me.
How do the eggs on day 14 handle the humidity of lockdown needed for the eggs on day 21?