I asked our OT therapist about this and she said that there are two requirement. First, that they are able to communicate the need to go, and second, that they have the self care capabilities like pulling down pants. We are waiting for the use of signs to click with her before we even attempt to potty train.

We started it at 3 1/2 and it took a year, but she finally is now taking herself to the potty & only very occasionally having an accident. She is still in Pull-Ups at night, but my older two NT kids took a while to grow out of bedwetting (I think they are just very sound sleepers).

We did not start potty training until age 3.5, and now at 5 we are almost there. It's been a long road to get here.
If you can, if you have a therapist enlist them for some help or support.
You can always begin with just some readiness skills like taking them to potty when you are changing the diaper. If you notice an interest you can go from there, but it may take some time. Best of luck.

Thank you all for answering I think we will let her set the pace. She talks a few things but not as far as letting us know things. So maybe when she's ready. We shall wait and see :)

My youngest and oldest we started at 18 months (oldest - Aspergers, youngest NT) and they both trained within 1 month of starting. My middle one (PDD-NOS) on the other hand we didn't start until 3 years old (he didn't walk until he was 2 and it's kind of hard to train a non walker and didn't talk until almost 4.) He didn't fully train until he was 7 1/2. It was a long hard battle. He would tell us and would sit but never seemed to grasp the concept of going. It was so heart breaking that he wanted to do it but just couldn't figure it out.