Depending on what the cracks look like you can tell wether or not they are stress cracks or just gelcoat crazing from age. Stress cracks are often long cracks, with other cracks running parallel to them, or a central point with cracks radiating from that point. Crazing is more random tiny cracks that have no pattern, go in every direction, if there are cracks perpendiculatr to each other its most likely crazing. This occurs from the gelcoat naturally shrinking over time, picture a dried up lake bed. It will help to wax the gelcoat to keep it from oxidizing, you can repair by regelcoating but you would want sand the cracks out to prevent transfer into the new gel.