The way we describe ages, including years or people’s ages, can sometimes seem strange if you consider that years beginning “20” are referred to as “21st Century” (and similarly, years starting “19” were the “20th Century”). There is a very logical reason for this, though: when it comes to age, and time that has passed, we assign numbers after a period of time is complete.
Where is the 21 in 20xx?
Any year beginning “20xx” is part of the 21st Century because they form the 21st set of hundred years in recorded time. When we reach 2100, time will have been recorded for 21 centuries. Consider the start of recorded time – the first century were the first hundred years of time, years through 0 to 99. At 100 years, the first century was completed, and the second century began (100 to 199 years).
This is the pattern we stick to – we refer to a period of time as the first, second, twentieth or other period while it is being completed. But a number denoting age is assigned after that period has been completed. So 2016, for example, comes within the time that the 21st hundred years are being completed (making it the 21st Century), but we will not give years the number 21xx until after those 2099 years are complete.
How years work for ages
The same principle is used when we talk about how old someone or something is. As with the first century, a child is in its first year until it turns 1, but we do not say it is 1 until after that year is complete. If you are 15 years old, for example, then you have completed 15 years of life. At the same time, after you turn 15 you are actually entering your 16th year.
Typically, this is how age is described – we describe an age within the period it is completing, but we label the age with a number that shows completed time.
Note that this is not something that gets generalised to all labels of time. When are talking about age, it is true, but when describing something like a course or schedule you will find the time period being completed as a name. So during college you can say you are Year 1, Year 2 or Year 3, for example, meaning that is the year you are currently completing. But when it comes to talking about centuries, and giving numbers for ages, keep in mind that the label we use to describe an ongoing period of time is different to the number we assign it! For more information about styles of writing dates in general, please also see this article.