Well, according to most Superman origin stories, Clark starts at the Planet at about 26. So if Jimmy started at the same time between the ages of 14-16, the age gap would be something like 10-12 years. Other origin stories have Clark starting at about 22, so that closes that gap to about 6-8 years. Either way, he'd still be older that Dick who generally doesn't enter Batman's life until the second or third year into his career at anywhere between 12-15 depending on the story.