What are the chances of teeth moving into the spot in which a bicuspid tooth was extracted ? 21 years of age ?

Low chances: Studies show that tooth movements occurred at a decreasing rate between 18-35 years of age. Your jaw stops growing around 18 years of age. That said, healthy teeth can be moved at any age with orthodontics. The odds that the gap would spontaneously close is very unlikely

Pretty good: There is a pretty good chance that your teeth will shift somewhat over an extended period of time. Posterior teeth tend to lean mesially and opposing teeth super-erupt into the space. Replacement would be wise to maintain a healthy mouth.

Uncontrolled: Adjacent and opposing teeth may, over time tip and extrude into the extraction area creating a pathologic condition called posterior bite collapse. Teeth can be Orthodontically moved into an extraction site with proper control. Another solution is implant/crown. See your Dentist, don't depend on Mother Nature, she never went to Dental School.