Final answer:
A greenstick fracture is least likely to occur in a 90-year-old due to the nature of elderly bones being more brittle and prone to compression fractures.
Step-by-step explanation:
A greenstick fracture, which is a type of fracture where the bone bends and cracks, only on one side, would be the least likely to occur in a 90-year-old individual. This type of fracture is most common in children, whose bones are more flexible. On the other hand, older adults are more prone to compression fractures due to reduced bone density and conditions such as osteoporosis. In the elderly, bones are more brittle and less likely to bend as they would in a greenstick fracture.