All of the answers you have mentioned are correct in usage. Here, the differences among prepositions indicate a subtle bias on the part of the speaker or writer.
Utlizing the prepostion "of" here, indicates a more formal distance from the listener or reader while "at" emphasizes the point of their lives where a certain event occurred, bringing the two discoursal participants into a closer social relationship. Without a preposition, the person is merely stating a fact.
Eric Paul Monroe