You are asking the wrong question. Don't worry about it...everybody does. Everybody wants to know when and nobody wants to hear: "When the dog is ready"...but that is the right answer.
It is not about the dog's age. When discussing training, it is almost NEVER about the dog's age.
If the right question is.."How do I know when the dog is ready for the e-collar?" then the answer might be...
When the dog has demonstrated that it understands the command, when it understands the necessity for instant compliance and when it has demonstrated that understanding by executing the command reliably and repeatedly in the yard when on a checkcord.
It is, or should be, about the level of training the dog has attained. If a dog is 10 months old heeling and whoaing and coming reliably in the yard on a checkcord, then it is probably time to overlay the e-collar and phase out the checkcord for those commands.
Conversely, if the dog is three years old and does not heel or whoa or come reliably in the yard, an e-collar will probably do more harm than good in the hands of most amateur trainers.
It helps if you think of the e-collar as nothing more than a very long checkcord. If the dog does not know what the commands are and does not know what you want him to do, the stimulation from the e-collar will probably only confuse him, or make him shut down.
Think what...not when and you will be on the right track. Training is, or should be generally task oriented and additive in nature and is not particularly time sensitive for most things.
PS - If you focus on getting the dog to perform the requred behaviors with precision and quick response, you will be pleaantly surprised at just how quickly you and the dog can often progress through the steps. Focus on getting the what in place and the when will take care of itself.