Fine-motor skills are basically the same regardless of your age, but most activities for developing them are focused on very young children who are just beginning to train their smallest muscles for future use in writing and other complex adult applications. Adults are likely to face fine-motor training when they've suffered injury to their hands, arms, or nervous systems, such as after a stroke. Although favourite activities from before the injury may be too frustrating, those activities may also incorporate useful starting points for rebuilding skills.
Simple Repetitive Exercises
Fine-motor skills can begin very simply, with counting on the fingers or learning finger spelling (see Resources). For strength training, keep a rubber band handy and stretch it between the fingers of one or both hands. Even twiddling your thumbs amounts to a fine-motor activity.
An Easier Version of a Favorite Craft
If you always liked embroidery but can't manage fine crewel work now, start with laced leather items, latch-hooked or punch-needle rug-making, or large-scale macrame. If your art was pen-and-ink drawing, try watercolour and explore your abstract side, or take up Chinese calligraphy.
Try a New Art
Start with the easier items in folded-paper origami and work up to something more impressive and challenging. Fine use of scissors is harder than it looks, so save it for an addition to simple folding. Take up sculpture, starting with soft modelling clays and putties.
Play With the Children in Your Life
Shoot marbles in a ring on the floor or into a box on a table. You might also build an extended and elaborate trail of dominoes that you and a child can have fun knocking down. Don't be afraid to discover or rediscover building toys, especially if you can share them with a child you love. It's the snapping together that works for your fine-motor skills, though, so don't get hung up on achieving an adult-looking final product.