I think you’ll find that most therapy dog organizations require a dog to be at least one year old, giving time for basic training to have occurred. But, that having been said, I always tell people – the older, the better. Starting the process too early can often be discouraging and end in aborting the idea completely.
Certain breeds mature more quickly then others. I think Poodles and Cavalier King Charles Spaniels were born knowing how to be therapy dogs – while Labs and English Setters are slow to mature and more challenging to train. And, within a certain breed there are differences. One person came to me to have his young Lab evaluated. Just over a year old, I was dubious. The dog amazed me – he was truly a natural! I complimented the owner on his obvious good training – and he replied, Thank you, but I can’t take credit for him being like this – he was born this way!
If at the age of one year, you feel your dog is ready, then by all means schedule an evaluation. If you are uncertain, I suggest you seek the opinion of a professional trainer. This is one of the services I offer folks with private instruction. It gives you the opportunity to see if in fact your dog is ready – before you apply for the formal evaluation process.