Add a column where you calculate the birthday for current year
Code:
=DATE(YEAR(TODAY()), MONTH(BirthDate), DAY(BirthDate))
or where you calculate date of next birthday
Code:
=DATE(YEAR(TODAY()) + (DATE(YEAR(TODAY()), MONTH(BirthDate), DAY(BirthDate))<TODAY()), MONTH(BirthDate), DAY(BirthDate))
and use the calculated column for sorting.