The chemistry and physics of strings - all materials - shows that as they age and stretch, they lose some of their dynamic properties that allow them to vibrate efficiently. They lose their elasticity. Since the role of strings is merely to vibrate (the actual sound production is the role of other components), then you should replace them when they are unable to do that at their best ability. Eight or nine years is well beyond their 'best before' date.
Plastic - including nylon and fluorocarbons - deteriotate under numerous environmental pressures including moisture and sunlight, making them sound flat and dull. Metal strings can be subject to corrosion and rust as well. All strings are subject to stretching, contamination from the oil and acids on your fingers, dust, friction and so on. Individual strings will also age and wear differently, so you can have a set with three good-soundng strings, but a fourth that's gone flat.
So the answer is: yes. Change them, and do so more frequently. I generally purchase more than one set at a time so I can have a spare or two handy.
Strings can last 1-2 years with care (cleaning, kept out of sunlight and away from excess moisture). But it depends on how often and how hard you play. With concert playing or recording sessions, they may last a single day. Many people change their strings every 3-6 months regardless of the sound. I tend to go to the 6-12 month cycle, but it depends on hoow the instrument sounds at any time.