。
She's old enough to travel alone.
Literal
She [topic-は] alone travel [permission-てもいい] age is.
The whole pre-noun stretch ひとりで旅行してもいい is a relative clause modifying 年齢 ('age') — literally 'an age at which traveling alone is permitted.' Japanese piles modifiers in front of the noun rather than splitting them off with 'who, that, which.' ~てもいい is the standard permission frame ('it's okay to do X'), and here it doubles as 'old enough to' because the 'permission' is licensed by age itself. ひとりで ('alone, by oneself') is a compound of 一人 + で, common in everything from solo travel to solo dining.