Why dont all people grow at the same time and at the same age?

Answer 1

In a word, genetics.

To be a bit more in depth, everyone is different, with different facial features, skin tones, hair lengths and thicknesses and other features. Our physical appearance is a result of the way that the chemicals that make us up interact. The mix of chemicals varies from person to person and from day to day, but how they vary is determined by

a) what we put into our bodies in terms of food, environment, drugs, etc.

b) what our body does with what we give it, which is determined by genetics.

Every person has a unique genotype which controls how their body processes chemicals and what it makes from the input. My body may make more androgen then yours, and as a result I am taller. Your body may make more testosterone, and as a result you have more hair.

This is a grossly oversimplified answer, but if you want more detail look into genetics as a field of study.

Answer 2

People grow at different rates and ages due to genetic factors, differences in hormone levels, and variations in nutrition and overall health. Growth spurts are also influenced by individual differences in puberty onset, which can occur earlier or later for each person. This variability results in people reaching their full height at different ages.