OB/GYN is one of the primary care residencies, and I do not believe they are particularly competitive. Of course, every specialty has residencies that are more competitive than others within the specialty. Most people are in their late 20s when they begin residency, but the average age of first year med students is increasing, so expect that the average age of interns will do the same. Most med students finish in four years. Those who take longer may have been in a combined degree program (MD/PhD, MD/MPH, MD/MBA, etc.), have taken an extra year for non-degree research, took a year off because of childbirth or illness, or were required to repeat a year due to inadequate academic performance.