Ken Nakagawa is a popular musician, member of the band "Manbou" (Mambō)*, where he is a vocalist, guitarist and songwriter. He is cameo character from another Ai Yazawa's manga "I'm Not an Angel" ("Tenshi Nanka Ja Nai").
He is around the same age as Midori Saejima and Shuuichii Takigawa, who also appeared in Gokinjo Monogatari, which means he is around 22 years old (6 years older than Mikako and her peers). And in the last, Extra chapter, then, he is around 33 years old.
Behind the scenes
*Band's name "Manbou" in japanese pronounced as "Mambō", (because ン[n] before ボ[b] and ぴ[p] usually reads as [m], as in a word えんぴつ [empitsu] - "pencil"). Also, in japanese マンボウ [Mambō] means Sunfish.
In different translations band's name spelled as Manbou and as Mambo.