Context matters...

In summary, Kiffin has a more established and successful record coaching exclusively at the highest level of college football (FBS) at this age. Kelly's higher overall winning percentage at the same age was largely built on his success in Division II.

Lane Kiffin, currently 50 years old, has a significantly higher winning percentage at this age in college football compared to Brian Kelly's record when he was the same age. Kelly was primarily a Division II coach at that point in his career, while Kiffin has a substantial Football Bowl Subdivision (FBS) record.
Coaching Records at Age 50
Coach Age (Years) Overall Head Coaching Record Winning Percentage (Overall) Primary Level at Age 50
Lane Kiffin 50 117–53 .688 FBS (College)
Brian Kelly 50 189–62–2 .751 Division II

Lane Kiffin, currently 50 years old, has a significantly higher winning percentage at this age in college football compared to Brian Kelly's record when he was the same age. Kelly was primarily a Division II coach at that point in his career, while Kiffin has a substantial Football Bowl Subdivision (FBS) record.
Coaching Records at Age 50
Coach Age (Years) Overall Head Coaching Record Winning Percentage (Overall) Primary Level at Age 50
Lane Kiffin 50 117–53 .688 FBS (College)
Brian Kelly 50 189–62–2 .751 Division II (College)
Lane Kiffin's record includes his time at Tennessee, USC, Florida Atlantic, and Ole Miss, predominantly at the FBS level. His NFL head coaching record (5–15) is excluded from the college comparison. Kiffin achieved this record with various major college football programs and led Ole Miss to four 10-win seasons and an 11-win regular season by age 50.
Brian Kelly's record at age 50 (around 2011) primarily reflects his highly successful tenure at Division II Grand Valley State (where he won two national championships) and the beginning of his time at Central Michigan and Cincinnati. By this age, he had just completed his second season at Notre Dame, where he went 8-5 and 10-2. His overall record at 50 was heavily influenced by his dominant D-II run.
In summary, Kiffin has a more established and successful record coaching exclusively at the highest level of college football (FBS) at this age. Kelly's higher overall winning percentage at the same age was largely built on his success in Division II.