I'm starting residency this July at age 36, and I will complete a fellowship as well for a total of 5-6 years. Length of training played no role in my decision. I love this field almost to the point of obsession, and I'd have still chosen it even if the training were ten years. That being said, it helps a lot that I have no loans (was scholarshipped to med school), and no children/house (so no large expenditures, either). If you have a quarter million dollars or more in loans, two or three kids, a house, etc, your decision may be significantly affected by practical considerations. In addition, your med school performance may also limit your options no matter what you want and how many years of training you're willing to do. So if you barely pass Step 1 and have no research experience, neurosurg will be a long shot, and you'd do well to have a backup plan.
The best advice I can give those of you who are just starting out is to be open-minded about specialty. Spend time shadowing your first year. Talk to the docs you meet about the pros and cons of their specialty. Pay attention to which preclinical subjects you find enjoyable to study, and where you feel like you fit in during your clinical rotations. You may be surprised by where you end up landing in the end. I sure was. 🙂