I don't think pain is a very strong predictor of disability - if at all. So try not to read into it that you're losing the use of your legs.
Unfortunately, I don't get a lot of interest from my neuro when I try to talk about pain that's NOT obviously neurological (tingling, burning, electric etc.) He always seems to take the view it's unconnected.
However, although I'm 45, I don't think it's very likely to have all these aches and pains, but it have nothing to do with MS. Other people my age don't complain of this all the time, and I don't think I did, either, until I started developing other symptoms of MS.
So to my mind, there's definitely a connection - even more so if you're just twenty, because why on earth would your knees start playing up, at twenty, if it weren't for the fact you're already ill?
MS can cause pain in a lot of ways - sometimes just because our gait and posture is altered, so we might be putting abnormal strain on joints and muscles. Over time, asking them to do what they're not designed for leads to pain.
You might get more help from a neuro-physiotherapist than from a neuro, because they are better at spotting postural abnormalities, and will be more clued-up about what the effects could be.