Men's height after age 20 tends to remain the same until after age 50 years, then a gradual decline in height occurs. The thick red line in the middle of the chart shows the "median" (50th percentile) height of men.

The red lines show "percentiles". The thick red line in the middle is the 50th percentile, which indicates that 50% of the population of Men have Height (or stature) taller than the line, and 50% are shorter. Similarly, the lowest red line, the 5th percentile line, indicates the Height where only 5% of the male population is shorter.