It was never confirmed on the show. I believe Eli was born in Nov. 2007 (at least according to the episode that featured his birth) so he should've just turned 14. The actor that plays Eli said the character is 15. I figured they aged him up a little to be at least a freshman in high school. To me, he comes across as 15/16. I guess by aging him up a little, they can go a little further with the storylines involving Eli.