What should my ideal weight be if my age is 21?
People come in all different shapes and sizes, carry their weight differently and have all manner of body frames. So it's important (and the Centers for Disease Control agrees with me) to assess your weight along with your height, which, together is called the Body Mass Index or BMI.
For example, say a woman weighs 105 lbs, but is 5' 8" tall. This would place her in the underweight category, whereas if she was 5' 2" her BMI would be in the "Normal" range. The link below is to the BMI calculator for the CDC.
Age is a factor in what is considered a healthy weight ratio, including in body fat percentage, and gender can make a difference too, but most nutritionists say BMI is the best measure.