dear emilyjanep
painful knees can be really annoying...i say this cause i know how much of a nuisance it can be to daily activities of life. howvwer , giving you some suggestions will be hazy considering the lack of detail in your history. you might want to expand more on your symptoms, i.e how it behaves in the morning, or at night...any added stiffness? did it come suddenly or gradually, what makes it better? what positions eases the pain,? is it a constant pain...?does ordinary bending and straightening the knee make it worse or is it only in weight bearing tasks? does it feel warm, any swelling? the list is endless...do you play any sports...
Nonetheless, in my own opinion knee pain made worse with using the stairs most often points to a patella maltracking problem...because of your age, it is unlikely that any degenerative changes are occurinng however this is not a true factor for total exclusion...your age however points to problems with growing bones...it is possible to develope problems with patella tracking from disruptions in how the proximal tibia forms...because this is were the patella ligament that goes over your knee cap attaches
having shin pain also adds to that possibility...if you are having problems putting weight on your knees then i would suspect issues with the bones. these are only speculations from what you have described...
your best bet is to have an x ray done first to rule out issues with growing bones, then have the patella tracking assessed...
best of luck