As an older learner it took considerably longer for me, over a year, but it did eventually happen — just wanted to add a data point for anyone worried age rules it out entirely.
It happened last week — I caught myself doing mental arithmetic in the local language without translating, and a few nights ago I apparently dreamed in it too. After months of feeling like I was getting nowhere, these little signs felt huge. When did your brain first flip like that, and did it feel as surreal to you?