Normal heart rate for a 21 year old female?

The normal heart rate for an adult is between 60-100 beats per minute, with the elderly and sedentary leaning toward the higher end of that range. You can check your heart rate by feeling for your pulse at the radial artery, which is on the inside of your wrist near the thumb - the artery goes over a bony prominence and is closer to the surface here, so it is easier to feel.
A heart rate lower than 60 can be found in very healthy athletic adults. Generally above 50.
Any exertion can increase the heart rate. There is a measurement called Target Heart Rate, which is what you should strive for during cardivascular activity, and is found based on a formula including your age and weight.
There are some heart abnormalities, such as dropped beats which can cause your heartbeat to become irregular. Generally as long as you are feeling healthy this isn't a concern, but in the presence of chest pain or weakness it can be an indicator of heart arrythmias and medical attention is indicated. A lot of people live their lives with heart arrythmias and it's not a problem for them, but it is something to be aware of.
All adult males over 35 years and adult females over 45 years should obtain a yearly Electrocardiogram and keep it with their medical information in the wallet or purse, as in an emergency it can be very useful to compare past ECG's with present ECG's.
Also some medications can affect your resting heart rate. Please read and remember the side-effect information on all prescription medications that you take or over the counter medications that you take regularly. -chris, certified paramedic and licensed EMT.