I don't think springs and leafs get firmer with age.
Along with wear-steps, as explained by Bill, rust between the leaves when it gets bad enough, will also stop the leaves from sliding past each other. This then changes the characteristics of the spring-pack to a rather unyielding block of steel.
To make sure that the springs slide/flex easily, they either need lubricating; separating with spacers such as plastic, or sometimes rubber; or in the case of parabolic springs, separated with a significant air-gap.
Anyone who owns an Alpine, really needs to know what condition the spring-packs are in; if they are rusty or dirty, or missing spacers, they really need to be stripped; inspected; repaired as per Bill's instruction and replaced. Even new, painted-springs do not always work well if they are not lubricated, or separated and radiused on the top edge. When I overhaul leaf springs I also narrow the end of the leaves by cutting a taper on both sides; this reduces the initial effort required by the spring to start sliding and is recommended by the Society of Automotive Engineers. It is also the ideal time to replace the spring bushes.
After I had done that I would start thinking about dampers,
Cheers Charlie