I'm an 18 year old female, 5'4, at 100.8 lbs. is that normal or underweight?

Depends: Really depends on body composition. One hundred pounds for 5'4 seems a little on the low side. However, if you feel good and are healthy and active I wouldn't be concerned about it.

U R LOW: YOUR WT FOR 5 FT 4 INSHOULD NOT BE BELOW 107.5 BASED ON BMI CALCULATIONS. MAX AT 5'4" IS APPROX 146 POUNDS...THE ONLY PLACE I LIKE TO SEE RIBS IS ON A PLATE!!! GET MY DRIFT??? YOU MAY COME FROM A FAMILY OF FAIRLY THIN ADOLESCENTS AND WILL BLOSSUM IN FUTURE YEARS..GOOD LUCK STAY HEALTHY EAT 3 MEALS A DAY AND TAKE A N MVI (multivitamin) DAILY

@3% for age: This places you at the lowest 3% of the population for your length with a BMI of 17.25 while the average BMI is 21.25 or about 124 lb.