ummmm didn't think there was an 'age' for them (they are pasturised) but they are dairy so you would want to be sure hollie is ok with cows milk - if she has formula she will be (i think!)
plum baby do some really nice organic ones, they have half the sugar of petit filous/little stars etc so maybe try those? i can't imagine a little bit of yogurt would do any harm but probably just a tiny bit at first.