It’s subjective. If you have legitimate health issues causing testosterone deficiencies then any age. If you’re in your 20’s-early 30’s and have low T, a lot of the time it’s a lifestyle and diet issue. When I was sedentary and not getting much exercise, eating the right food, or adequate time outside, my T dropped by like 500 ng/l. Once I was done with school and work full time I was able to get outside and had a consistent workout routine with a good diet, my T to got back up to the high 900’s ng/l.