I have bad knee pain, doctor said it might be my meniscus and said it might be the cartilage. does that seem right?

In a word, yes: Bad knee pain can be from a variety of sources, one of which may be meniscus or cartilage. Other possibilities include a ligament tear, bone spur, arthritis (not common in your age group), gout, autoimmune, infection, muscular spasm... in short, quite a few things. Also, look at your feet and your hips. Sometimes, if a joint above or below is misaligned, bad shoes, etc., can cause knee pain.

Meniscus: is cartilage in the knee joint. There is a medial and a lateral meniscus they sit side by side. Ligaments hold everything together. I don't know whats going on in your knee, but if it locks up when bent and hurts a lot its probably a problem with the meniscus. Ligaments can tear. There is also the possibility of a small tibial plateau fracture. I'd find out and get the proper treatment.

Yes,: You'd need an MRI and arthroscopy to know for sure, unless you have other more definitive symptoms

Yes: depending on your symptoms yes it could be related