Knee caps

Question
I was wondering if I should be concerned about my knees, (and I'm guessing more specifically my knee caps) they seem to get inflammed or swollen and when I bend my legs and theres something popping and there is a prickling pain in specific spots after I run. I know that the kind of shoe you wear affects runner's knee and my shoes are light and they are about 3 years old. So theyre not the best shoes to run in, but I actually run on the grass that is right next to the sidewalk for the very reason of trying to prevent my legs from getting hurt. Is running on the grass not enough and I need new shoes? There's nothing really softer to run on than grass. It seems like my knees started hurting after I began to do interval training (like sprinting for 30sec. and running normally and repeating this), should I stop doing that? Thanks for the help  

Answer
Genna,

First of all shoes are usually good for about 500 miles. This being said, you should most likely get some new ones for sure. Running on grass is excellent for you and highly recommended, however, running on grass will not make up for badly worn shoes.When shoes wear out, your form changes likely leading to problems.