Manufacturers typically recommend that you replace your double mattress every 10 years or so for consistent night-time comfort. Coil-spring and padding density dictate the firmness and support it provides.
Choose a soft mattress with more padding for snuggly comfort, but seek a supportive orthopaedic version with higher coil spring density for added support. Alternatively, memory foam conforms to the body, and manufacturers claim that this provides a more natural body alignment. Make a firm double mattress decision and sleep in blissful comfort.