There's a lot of different forces acting on your knees when you run. Knees are often a source of pain, but the root cause can be somewhere else .
One of the most common causes is from the running shoes, are you wearing running shoes or a pair of trainers/plimsolls picked up from a sports shop? If theres a running shop nearby, it may be worth going in and having a gait analysis and listening to their shoe recommendations. The analysis is normally free, as they recoup the time/cost from shoe sales.
A more expensive, but more comprehensive assessment can be done by seeing a sports therapist or physiotherapist. They can look at your bio-mechanics to try and identify weak areas that are putting exra stress on your knees, and causing the pain.
Don't give up! Nearly everyone can run when they are a small child, but if you've got out of practice between your teens and now (like most people do!) then it takes a while for your body to adapt.