Yukiko is an innocent young girl, not yet old enough to know better.
Literal
Yukiko [topic-は] age [even-も] not-going innocent girl [copula-です].
年端も行かない ('not yet reached the age') is a set phrase for someone very young — too young for the thing being discussed. 無邪気 (むじゃき, 'innocent, guileless') is the key descriptor. The construction 年端もいかない is somewhat literary and emphasizes the child's youth with a nuance of 'not even at the age where...'