Knees problem because hiking too hard?

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I like hiking. If we have chance, we will go hiking every weekend. Durning the weekdays, i play badminton twice a week. I feel good after doing all the excites. But things have change after my knees start act funny. It was not a big pain, but it start making me worry, i heard from some of my hiker friends,when they are young, they can go miles and miles, but when they getting oldder, they wanted to go further, but their kness did not allow they to, because they felt pain on their kness if they get further. They told me to protect my kness when i was young, i should always keep my knees warm, don't over load when we are going down of the mountains, etc. I am wondering if any of you have such problems now? If so, how do you deal with it? If not, any good suggestions? Thank you for your post!