。
She isn't old enough to travel abroad on her own yet.
Literal
She [topic-は] alone [as-で] foreign-travel do [extent-ほど] [of-の] age [to-に] hasn't-become.
ほど sets a threshold — '(an age) to the extent of doing X.' Combined with の to attach to the noun 年, the phrase 一人で外国旅行するほどの年 reads literally 'an age sufficient for solo foreign travel.' The negative になっていない is a perfective negative: 'has not yet reached that state.' 外国旅行 ('foreign-country travel') is one of countless N+N compounds in Japanese — the language builds new concepts mainly by stacking nouns rather than borrowing.