Everything Nasa builds is flimsy, the ISS is no exception. In 2015 the core modules of the ISS will be approaching 20 years in age or older. They'll be worn out, and there's no way of replacing them while maintaining the stations structural integrity.

In the original design, the structual backbone of the station was the truss system. The modules could be unplugged and replaced. On the ISS that's not true. The Russian Zarya and Zvezda modules are the core and backbone of the ISS. They are so integrated into the station that the station's effective lifespan is the same as these two modules. A couple of excerpts from the link above:

The Zvezda service module of the International Space Station had its origins a quarter century before it was launched. The Mir-2 space station was originally authorized in the February 1976 resolution setting forth plans for development of third generation Soviet space systems. It would undergo many changes over the years, with only one thing remaining constant: the starting point was always the DOS-8 base block space station core module, built as a back-up to the DOS-7 base block used in the Mir station. When the International Space Station was agreed to, the $3-billion DOS-8 became the first Russian segment of the station, originally known as the Service Module.

That means that the Zvezda module is as old as the components for Mir.

DOS-8 serial number 128 was originally designed to have a three year service life in space. This was later increased to five years. The spaceframe was completed in February 1985 and major internal equipment was installed by October 1986. Until Reagan's 1983 announcement of the Star Wars program, Mir-2 was to be a relatively modest station, a near-duplicate of Mir assembled after the end of its planned five year life.

The lifespan was expected to be five years. It was launched in 1999...7 years ago. In 2015 it will have been in orbit for 16 years. That's 3 times its estimated service life. I don't think the ISS will make it to 2015.