Msg 156, Level 15, State 1, Procedure ISDOB, Line 4 Incorrect syntax near the keyword 'DECLARE'.

Create function [dbo].[ISDOB](@colname VarChar(100))
RETURNS VarChar(100)
as
BEGIN
DECLARE @From_Date date = '1980-01-01';
 DECLARE @To_Date date = getdate();
 DECLARE @REC_Date date = @colname;
 IF  @REC_Date > @From_Date AND @REC_Date < @To_Date
  return 1;
  return 0;
 END