Well, babies under the age of 3 months have first blue eyes. The eye color changes into the final colour after about 3 months.
So, maybe Regina was around 3 months old.
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"Love is a mystery to be uncovered." - Belle
Keeper of: Belle's Love for Rumple,her Hair and her Eyes,Rumple's Eyes,his Call to Belle and his Dreamcatcher,Snow's Heart, Emma's enjoying Magic,Regina's Castle