I also think the solution of Mike.Cornflake is the best idea. But if you still want to use a simple edit, do masking or use this:
procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: char);
var KeyLength : byte;
begin
KeyLength := length((Sender as TEdit).Text);
case KeyLength of
0 : if not (key in ['0','1',#8]) then key := #0;
1 : if not (key in ['0'..'9',#8]) then key := #0;
else if not (key = #8) then key := #0;
end;
end;