Not quite an old wives tale but perhaps rather exaggerated. Mackeson ("and by golly it does you good") used to be recommended for nursing mothers and even given out in maternity homes, but I suspect only had much effect on the malnourished. There is indeed iron in beer; a pretty small amount compared to daily needs but there really is more in dark beers than light (Scientific proof here). You would need to drink quite a lot to make a substantial difference though. Well that's one excuse...