How To Boost Testosterone After 40:-As men age, testosterone levels naturally begin to decline, usually starting in their 30s and becoming more noticeable after 40. Testosterone is an essential hormone responsible for muscle mass, energy, mood, bone strength, and sexual health. Lower levels can lead to fatigue, reduced libido, weight gain, and decreased motivation. Fortunately, there are several natural ways to support healthy testosterone levels and improve overall well-being after 40.