This is still an under researched field (with a lot of alternative medicine types surrounding it), but there have been some studies that even relatively minor antibiotic usage in childhood may have long-term impacts. And there are more (and better) studies that gut flora isn't back to normal 6 months after a course of antibiotics. It's something of an open question currently. It seems like there may be a few stable microbiomes that can develop in humans; some are better than others for health.