From the size of the trees I am supposing these are relative new plantings? The Cimmaron Ash, a Fraxinus Pennsylvanica cultivar generally holds its leaves a few days later than most of the species. However, many climatic and cultural conditions will cause variances.
Do post pictures so we can see if anything other than a natural fall abscission is taking place.
A good rule of thumb is to look around your neighborhood and see what other trees are losing their leaves. However, younger, newly planted trees will react differently until fully acclimated.
Sylvia