In the US, you may be surprised to learn that many people think you’re older when you’re still in your 20s or 30s.
But is it really that simple?
The reason people think older is that the amount of biological clock that tells us we are older is set to reset.
It is set by our own genetic clock and our genes.
This is why our clocks are ticking.
So it’s not surprising to see people start to think they are younger.
But it’s quite different to be told that you’re a little bit older than you really are.
The idea that you are more likely to be in your late 20s, 30s or 40s is also based on the way that genes tell our bodies how to function.
The amount of clock on our chromosomes tells us what age we are and what we will get, so if we were told that we were 30, it would be a little hard to believe that we would be.
But the clock has reset and so people now believe they are older.
Why are people so sure that they are 30?
There are two main explanations.
First, some people have more genes that tell them to be older than others.
For example, some of the same genes that are responsible for making you a man also make you a woman.
Second, it is easier to believe older when it comes from someone with more than one parent.
What do you think?
Does the clock just stop when you are older?
Or are people still really confident that they’re 30?
More: What does age mean for us?
Read more about how our genes tell us when to go to work, when to have children and what happens to us after we die.
Read more from the BBC