We don't know, as it is unclear how many regenerations she went through before becoming the woman known as River Song.
Even if we had a complete & accurate timeline for River (which we don't), it would depend on which episode you're asking about. For obvious reasons (she dies), she's at her oldest the first time we see her, in "Silence in the Library"/"Forest of the Dead". Subsequent appearances take place at various different points in her timeline in a fairly random sequence, so she may be younger in one story than she was in a story we saw earlier. In some stories, we've seen her at different ages in the same episode. In "A Good Man Goes to War", for example, we saw her as both a young baby & as an adult.
In "The Husbands of River Song", she said she was 200 years old.
The actress Alex Kingston turned 50 in March 2013.