Because 少年 in hiragana is しょうねん: spelled out that is "sho u ne n"

Many, but not all long vowels in japanese follow these:

ああ a i -> as in おかあさん, mother

いい i i -> as in ちいさい, small

うう u u -> as in すう, to smoke

えい e i -> as in せんせい, a teacher

おう o u -> as in こうえん, a park

Yes, exceptions to this exist (like おお) and some are actually dipthongs and not actually long-vowels, but easier to think of them like that.