Which of the following is considered correct? ". . . where I was at age five." or ". . . where I was at age 5."
1Neither is considered correct (except in very constrained circumstances you're probably not interested in). For more information, please ask any follow-up questions on our sister site, English Language Learners.– Dan BronApr 20, 2016 at 15:21
This would be a matter of style. Different style guides and audiences expect different formats. For example, some style guides specify that numbers under 100 be actually spelled out ("ninety-nine"). Other style guides do not require this. I would make sure you know who your audience is and what they expect to see.