As thumbs age, they naturally undergo changes like osteoarthritis. Proper "posture" and care can prevent these changes from becoming deformities:
Some thumb features are purely genetic and cannot be "developed" through exercise:
: Adding thick grips to dumbbells or barbells during pull-ups, rows, or deadlifts forces the thumb and hand muscles to work harder to maintain a hold.
The "mature" look of a thumb often comes from a well-developed (the fleshy muscle at the base). You can increase this size through high-resistance training: