also: としはもゆかない
expression, i-adjective
of tender years; very young
Describes a child who is still very young, often implying innocence or lack of experience. Commonly used in the negative form 年端もいかない (lit. 'not yet having reached many years').
See also: 年端もいかぬ
年端もいかない子どもにそんな重い責任は負わせられない。
You can't place such a heavy responsibility on a child of tender years.
彼女はまだ年
She is still very young, but she is very mature.
From 年端 (としは, 'years of age') + も (inclusive particle) + いかない (negative form of 行く 'to go/reach'), literally 'not yet reaching many years'. The phrase is idiomatic and its exact origin is uncertain.