I introduced DS1 to yoghurt at about the 9-10 month mark. Just plain, natural yogurt, unsweetened, which he was happy to eat at that age. I'm wary of the "kids" yoghurts as I think they tend to have more sugar in them, and it's a bit of a marketing gimmick anyway. A good quality pot-set yoghurt with active bacteria thingies (like Jalna) is a good place to start.