What is a significant physical development characteristic at young adulthood (18 to 40 years)?

In young adulthood, typically defined as the ages between 18 to 40 years, individuals have usually reached their full adult height. This stage marks a time when growth plates have closed, and stature has stabilized, indicating that height development is complete. This period also encompasses peak physical condition and fitness for many, as muscle strength, endurance, and overall physical capabilities are often at their highest.

The other options do not reflect the typical characteristics of young adulthood. For instance, while changes in body systems may occur, they are not still developing as they are in earlier years—most systems reach maturity by this age. Additionally, proponents of higher muscle mass and fitness levels can be highlighted during this stage compared to adolescence, when muscle mass is often lower. Therefore, the statement about adult height being reached accurately reflects the significant physical development characteristic typical to this life stage.