Good question. One example I can think of right now:

• X is told that he/she is too old to do exercise, and believes it.
• X gradually withdraws from physical activity, and incurs a loss of agility.
• X's lack of movement increases the rate of osteoporosis.
• As a result, X experiences a spinal fracture, causing even more immobility and perceived oldness.

Reference for the relationship between osteoporosis and physical activity. Does this make sense?