I read this in a grammar book recently but I can't recall what the specific term (if any) is called for something like:
He was 18 years old.
I don't remember if it is the same example, but the idea is generally similar. The text is written in past tense, so the example above should also be written in past tense. However, since he is currently 18 years old, not formerly 18 years old, the grammar book stated that it should be written as:
He is 18 years old.
Even though the text is written in past tense. Again, the example might be incorrect and my recollection might be a bit off, but the general idea is there.