How much should a 25 year old weigh that is 5'4?

According to BMI, Body Mass Index, a 25-year old's weight should be about 51 kg to 56 kg.

A way to calculate your BMI is weight (in kg)/height*height(in metres)

eg: 40kg/1.5m*1.5m
=40kg/2.25m
=17.78 (approx)

Normal BMI for a growing child is 17 to 24.9
Normal BMI for an adult is 18.5 to 24.9

You can also check your BMI at the following site: http://www.nhlbisupport.com/BMI/bmi-m.htm