People usually before age 20, can be clearly distinguished from the approximate age appearance. For example, 10 years old, 12 years old, 15 years old, 18 years old, there will be significant changes this time and catch up with puberty, it is easy to distinguish from the approximate age appearance. And 20 years later, in theory, to distinguish from the appearance of age, such as 20-year-old, 25-year-old, 30-year-old will have different characteristics. However, because many adults smoke, leading to accelerated aging, so there are a lot of people look older by 5 to 10 years younger than their peers who do not smoke, or even more. A 25-year-old long-term smokers, looks like a 30-year-old to 35 years of age; a 35-year-old long-term smokers, looks like a 40-year-old to 45 years old.

Smoking is terrible, you can "smoke aging" in the Google search, aging is because cigarettes will weaken the regeneration of human cells, repair capacity. If a person they do not smoke, but long-term air in public places have cigarettes, belongs to passive smoking, will accelerate aging.

Of course, people accelerated aging in addition to smoking, there are many other possible factors, such as staying up late, do not follow the schedule of life; a negative mood, and so on. Thus, after 20 years of age, the same age who do not look like their peers.