。
She's old enough to know the truth now.
Literal
[topic-は] now real [genitive-の] thing [object-を] know-[permission-てもいい] age [copula-だ].
本当のこと literally 'real thing' is the standard phrase for 'the truth' — Japanese reaches for the abstract noun 真実 only in more formal contexts. The construction 知ってもいい歳だ uses ~てもいい ('it's okay to do X') as a prenominal modifier of 歳 ('age'), giving 'an age at which it's okay to know' — the speaker is saying she's no longer too young to be told. Implicit but unstated: someone has been kept in the dark, and the speaker is now arguing that the protective silence should end.