I don't think that there is a certain age that they have to be. I am actually attending my god-daughter's baptism this Sunday and she is 6 months old. I think that it is pretty much up to you, but check with the church to make sure.
There's no right age, its whenever you're ready! But I agree the yonger they are the better they behave.I had Brayden baptized at 6 weeks and it was just perfect! But there was a two year old being baptized the same time and she screamed the entire time!