grammar - In a story written in past tense, is using present tense grammatically correct in the narration?

For example, just something quickly made up:

Sam started to run from the house to the nearby forest. The freezing weather caused him to shiver, but the warmth from running very rapidly heated up him. Sam, son of a lumberjack, is/was 16-years-old this year.

Given a context like that or similar to that, for the bolded text, which is one is correct? On one hand, "was" seems to be the correct one as the narration is in past tense, so for consistency, everything should be in past. But on the other hand, "is" also seems to be correct because he isn't "was 16-year-old", he is 16-years-old.

Thanks in advance for any answers, and if possible, please point to a resource I can study.