Maximum heart rate can be estimated using the formula 207 - (Age)(0.7) and target heart rate for vigorous exercise should be around 70 - 85% of this maximum.
For example, a 20 year old would have a max heart rate of roughly 207 - (20)(0.7) or 193 beats per minute (bpm). Their target heart rate is around (193)(.70) - (193)(.85) or 135 to 164 bpm.