In my experience, if the children of the testator/testatrix are listed one after another (e.g. to my children Elizabeth, Thomas, Sarah etc.), then they are usually listed in either age order, or sons followed by daughters. However, if they are mentioned separately in different bequests I would say it isn't that unusual for them to be just in some random order. I've certainly had a number of wills that mention specific children first that are not the eldest son/daughter. Sometimes daughters might be grouped together by their marital status as well.
I've not thought about it much, but my feeling would be that siblings might be less likely to be ordered by age than children.