Divergent: Caleb and Tris' Age Difference

If you're reading this post, you're probably just one in the millions of readers of Divergent who's confused with Caleb and Tris' age difference. Oh, no need to deny because you're not alone.

In the book, Caleb is said to be older than Tris but if he's older then how on earth did they take the Aptitude Test concurrently? So, here's my Analysis.

Let's say for example that the Aptitude Test is set on July 1, 3012, then Tris must have turned 16 before the exam while Caleb is already 16 and he will be 17 after the exam. Caleb and Tris must be Irish Twins. I know that the probability of my analysis is little but I hope it helped you're bewildered minds.