I feel like people need to understand how Japanese school years work. Boomshakaking and CKT0304 got it right: Nasa was in the latter stages of his 15th year of life since he was finishing up his third year of middle school. Four months in a coma and he'd be within spitting distance of his birthday on May 5th, and turning 16. Two years later would put him at 18 and availability to marry and fulfill the promise made. So he spent two years working and searching.
It's been staring everybody in the face since the start, but people kept underestimating how old he was when the accident happened, and how close to his birthday he was.
Technically, when they met, Tsukasa was just barely physically older than him, by the way, since she has consistently said and been depicted to be 16 in just about every instance of showing or talking about the past, such as when Tsukasa first met Chitose years ago.
All evidence has pointed to her aging process stopping in her 16th year of life at some point in the distant past. Not understanding this means you haven't taken note of the clues.