I answered a similar post the other day. Here's the formula I use. Here you would substitute the [Start Date] for a [Date of Birth] field.

var number currMonth=Month(Today());

var number currDay=Day(Today());

var number startMonth=Month([Start Date]);

var number startDay=Day([Start Date]);

Case(true,

$currMonth = $startMonth and $startDay <= $currDay, (ToDays(Today()-[Start Date])/365),

$currMonth = $startMonth and $startDay > $currDay, (ToDays(Today()-[Start Date])-365)/365,

$startMonth < $currMonth,Floor(ToDays(Today()-[Start Date])/365),

$startMonth > $currMonth,(ToDays(Today()-[Start Date])-365)/365

)