I would have to guess that the vast majority of shooters were introduced to shooting through a mentor. Whether that mentor was a dad, mom, grandfather, aunt, uncle or other family member or just a friend, the introduction to handling a firearm has often been learned from an informal mentor of some sort.
A mentor can be a great way to enter shooting. The informal and relaxed environment can make learning an efficient and enjoyable experience. The ability to proceed at your own pace and ask questions are all great benefits of having a mentor. One of the best advantages of a mentor is that it is not a one-time event. Typically, a mentor can and will continue to guide you over a long period of time. They can introduce you to the shooting community and be the one to answer the many questions you will have as you grow in your shooting experience.
With even the best mentor, you should consider taking some formal training classes once you have gotten past your first few shooting experiences. Formal training provides a different perspective and a structured approach which will enhance and reinforce the habits you have started to develop. Instructors, who have helped hundreds or thousands or students learn to shoot, can often provide solutions to shooting issues that a mentor might not identify.
Even very experienced shooters continue to take training classes often from different instructors because they have learned the value of different perspectives and techniques.