Two of my dogs have received their CGC certificates and the training did not take away any of their happy personality. I've trained my dogs to sit while being petted and if the person wants more, they are welcome to ask the dog to jump up or get excited.
Your dog can take their CGC test before a year. My youngest passed his at 11 months. There is no age limit.
AKC's Canine Good Citizen (CGC) Program