The formula (in column F) below does a nice job at providing me with
someone's current age (e.g. if Jan 24, 1988 is placed in cell E4, than cell
F4 would generate the person's age of 21).

=IF(E4="","",IF(DATEDIF(E4,TODAY(),"y")<1,DATEDIF( E4,TODAY(),"m")&"
months",DATEDIF(E4,TODAY(),"y")))

What I would like to do now is add a "Birthday Flag" type reminder. My
friend owns a fitness center, and he would like to send out a letter to his
client's on their birthdays. So, I need some type of formula that might
generate a "flag" in Column G when the birthday comes around (next to the
Column F current age column) stating "Happy Birthday-Send Letter".

I suspect that this flag would trigger off of the birthdate, entered into
Column E, however I just can't seem to wrap my brain around HOW to set this
type of formula up. Any help would be greatly appreciated:

Column E Column F Column G

Jan 24, 1988 21 Happy Birthday-Send Letter