When speaking of age 13 and over, is it 12+ or 13+?

When you take into account all ages above a certain age, what is the correct way to write it?

I was supposed to write all ages above 12, and not including 12, so I wrote '13+', which in my mind seemed to mean '13 and plus'/'13 and above'.

Someone told me it's meant to be written as '12+', but to me, this looks as if it's including the age 12 in it.

What is the correct way, when talking of all ages above but not including 12, to write it: 12+ or 13+?

The correct way, is 13+. People see that as "Age 13 and above are allowed / permitted".

If you're referring to something like the MPAA's PG-13 rating, it excludes children under 13, and includes movie-goers 13 and over.

So yes, you would write "13+" to mean ages 13 and over.

13+. It's an inclusive statement. However, like all moments where there's confusion for any reason, it might make sense to use something less ambiguous, such as your own "age 13 and over".