If we assume they are about to get off the island, after about 100 island days, then at least 9 months would have elapsed in the outside world - assumes it is plausible to the outside world that Kate was 3 months pregnant on the plane.

At least 12 months would have elapsed in the outside world if the "story" is that Kate conceived on the island. It could be more than 12 months, as Kate could be assumed to have conceived at any point while on the island. So I would guess that outside world time elapsed is 4 to 10 times island time elapsed.

Aaron's current age does not mean anything, as we do not know how long they have been off the island at time of trial - only his age at rescue matters with respect to inventing a plausible story.

This could be the reason Kate has custody of Aaron. If the islanders were made aware of the time difference before rescue and had to invent a story accordingly, then Claire could not claim Aaron as her baby, because he would not appear to be old enough. A baby that age could only be explained by saying that he was conceived after the crash (3 month old baby, world view that crash happened 1+ year ago).