I think that you need to ammend that to:
Record Selection Formula
{Table.BIRTHDATE} >= Date((Year ({?StartDate})-65),Month ({?StartDate}) ,Day ({?StartDate})) and
{Table.BIRTHDATE} <= Date ((Year ({?EndDate})-65),Month ({?EndDate}) ,Day ({?EndDate}))
If the table.birthdate is a datetime field, then you might want to use a datetime to assure SQL pass through:
{Table.BIRTHDATE} >= Datetime((Year ({?StartDate})-65),Month ({?StartDate}) ,Day ({?StartDate}),0,0,0) and
{Table.BIRTHDATE} <= Datetime ((Year ({?EndDate})-65),Month ({?EndDate}) ,Day ({?EndDate}),23,59,59)
-k