That's a great question, and as you can see from the replies, there's no single answer. While many people use rules of thumb like the "half your age plus seven" rule, a truly "big" age gap is often defined less by a number and more by the "life gap" between two people.<br /><br />For example, a five-year age difference is a very different experience for a 20-year-old and a 25-year-old compared to a 40-year-old and a 45-year-old. The biggest challenges often arise when people are in drastically different life stages, like one person starting a family while the other is looking to retire.<br /><br />Ultimately, the most important factors are mutual respect, shared values, and a similar level of emotional maturity. When those are in place, the age gap often becomes less significant.