Mo2 makes an interesting point because, at this age, you certainly need to take into account the personality of the child. I took DS to 'Aquababies' from 4 months until about 15 months at which point he refused point blank to do any of the lesson bits (not really lesson but more than splashing around IFYKWIM). The teacher taught lots of children from as young as 12 weeks who were swimming properly by 2yrs so it's definitely possible.
DS is now 2.5 and has an amazing confidence in water, will happily go underwater, in deep pools etc with no armbands but he certainly can't swim and I have to be really careful because actually he is much more confident than he should be given that he can't swim! I take him every week and we just splash about and he jumps in etc but that's about it.
I am going to try him with lessons again when he's about 4 but I think he I try any earlier he just won't want to be told what to do and it'll be a bit of a fruitless excercise.
If I were you twiglett I'd just try taking him yourself for a while and see if he likes it.