grow old in vain, namely be immature or childish. You should'nt use 처먹다, because it's very rude and impolite word in Korean.

나이를 헛으로 먹다 = "know nothing"...it's quite offensive. 나이를 헛으로 "처"먹다 = "처" makes this sentence very offensive.

ㅋㅋㅋㅋ that means sombody learned nothing through life