Age as a fraction. Thanks to http://blog.plover.com/math/age-fraction-2.html

age.sh

#!/bin/bash
BIRTHDAY=$1
DAYSTODAY=$(date +%j | sed "s/^0*//g")
DAYSATBIRTH=$(date -d "$BIRTHDAY" +%j | sed "s/^0*//g")
YEARSOLD=$(($(date +%Y)-$(date -d "$BIRTHDAY" +%Y)))
if [ $DAYSATBIRTH  -gt $DAYSTODAY ]; then
 YEARSOLD=$(($YEARSOLD-1));
 DAYSSINCE=$(($((365-$DAYSATBIRTH))+$DAYSTODAY))
else
 DAYSSINCE=$(($DAYSTODAY-$DAYSATBIRTH))
fi
if [ $DAYSSINCE -eq 0 ]; then
 echo "$YEARSOLD years old."
 exit
fi
isin() {
 isle $1 $2 $3 $4 && islt $3 $4 $5 $6
}
islt() {
 test $(($1*$4)) -lt $(($2*$3))
}
isle() {
 test $(($1*$4)) -le $(($2*$3))
}
LOWN=0;
LOWD=1;
HIGHN=1;
HIGHD=0;
while true; do
 NUM=$((LOWN+HIGHN))
 DEN=$((LOWD+HIGHD))
 if isin $DAYSSINCE 365 $NUM $DEN $(($DAYSSINCE+1)) 365; then
  echo "$YEARSOLD $NUM/$DEN years old."
  break
 elif islt $NUM $DEN $DAYSSINCE 365; then
  LOWN=$NUM;
  LOWD=$DEN;
 elif isle $(($DAYSSINCE+1)) 365 $NUM $DEN; then
  HIGHN=$NUM;
  HIGHD=$DEN;
 fi
done