I am experiencing pain on my kness and hands at my age around 29.What do you think it may be?

Many possibilities. Early onset of pain in multiple joints can be suggestive of an auto imune disease, such as rheumatoid arthritis or lupus. Family history can be a strong predictor. You need an exam and lab work.
Varies. There are multiple potential causes, a good medical checkup would probably be a good place to start.