Thanks for the replies. I settled on the following:

convert(date,

case when substring(a.IDNumber,1,2)<=substring(convert(varchar,getdate(),112),3,2) -- when ID birthdate year is less than current year

then convert(varchar,substring(convert(varchar,getdate(),112),1,2))+substring(a.IDNumber,1,6) -- then add current Centuary

else convert(varchar,(substring(convert(varchar,getdate(),112),1,2)-1))+substring(a.IDNumber,1,6) -- otherwise add prior Centuary

end) as BirthDate

This dynamically determines what century to add to the YY value based on the the YY of Getdate(). It does assume though that the dates being converted relate to the 100 years prior to getdate().