When you have a full head of gray hair what song

part, iii. Lyrics excerpt, when your head is full of gray hair, it is you who pushes the wheelchair to take me away, to the park to square dance and other old man, I have long guessed that I stayed a hand, pulled out the water gun and began to biubiubiu to you. from the part, iii song sung by mishi in the album "part, iii".