You can use the =datedif() function.

You can find lots of information about =datedif() at Chip Pearson's site:
http://www.cpearson.com/excel/datedif.htm

Craig wrote:

If I have a column with the date of births of a group of individuals how can
I determine their age (in the adjacent column) at their next birthday or as
of 1/1/2005?

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Dave Peterson