1 day, if one kid just turned 14 today while the other kid turns 14 tomorrow.
For example, today is July 20, 2019. You'd be 14 today if you were born on 7/20/2005... and 13 if you were born 1 day later on 7/21/2005
1 year and 364 days, if one kid just turned 13 today, while the other kid turns 15 tomorrow
For example, if one kid was born on 7/20/2006, he'd be 13 today. However, if someone was born on 7/21/2004, he's still 14 today.
In other words, it could be 14 years and 0 days, versus 13 years and 364 days
Or it could be 14 years and 364 days, versus 13 years and 0 days
If you're talking about a social or psychological difference (as opposed to a temporal difference)... there is a lot of growing and maturing that can happen in 12 months. Since kids absorb so much new information and perspectives every day... someone may be capable of thinking, feeling, and understanding profound truths about society and mankind that would've completely flown over their heads 12 months ago