Is age 21 too young to begin running marathons? i've heard my cartilage may be affected in the future. what is an "appropriate" age?

No: You are good to go. Work up slowly, gradually increasing miles/ week. I suggest a book on distance running and a runners magazine for proven, healthy tips.

You should be fine: Need to train and practice proper nutrition at any age. You may want to join a marathon training group or club to help. Also, consider cross training- bike, swim, yoga, etc.

21 is fine: The most successful age seems to be from the 20's into the mid 30's as far as competition goes.Marathon training involves a lot and much is gained from experience, thus the success at older ages. However, 21 is a great age to begin-- you have youth, strength, hormones, and curiosity all on your side.Train well and wisely and follow the wisdom of others.Do not be in a hurry, be patient.Best of luck

Not at all: At age 18, it's usually safe to run marathons.