My son has developmental delay (gross motor skills) and speech delay and he was trained at around 2yrs9mo. We gave it a try because the preschool really wants them trained, so we thought we would give it a shot. It was a huge fail. Tons and tons of accidents - he didn't care at all or have any idea that he was going to pee. I decided to go back to diapers and try again later - but he started absolutely freaking out if we put them on him. So we just kept going with it since it was easier than him having meltdowns because he was wearing a diaper.
It took a month for him to start holding it until I put him on the toilet, another month for him to be aware, and the third month for him to let us know. He's 3yrs 2mo now. He still needs help getting in and out of his pants, and we haven't thought about night training at all so he's still in diapers at night.
His understanding isn't delayed however - he understood the whole process. Our main issues were that he was resistant to doing it, didn't care at all if he was sitting in wet/dirty underwear, and him being able to communicate that he needed to go.