Errata ....

I wrote:
IF(DATEDIF(B1, "12/31/2004", "y") < 18, 0,
INT(MIN(H1, G1*3%))
+ 0.75*MAX(0, INT(MIN(H1-G1*3%, G1*6%)))
+ 0.50*MAX(0, INT(MIN(H1-G1*6%, G1*10%))))

Should be:

IF(DATEDIF(B1, "12/31/2004", "y") < 18, 0,
INT(MIN(H1, G1*3%))
+ 0.75*MAX(0, INT(MIN(H1, G1*6%) - G1*3%))
+ 0.50*MAX(0, INT(MIN(H1, G1*10%) - G1*6%)))