I have a mystery that I'm trying to resolve. I ride, for the most part, 5 to 6 days a week and occasionally my 18 year old son will accompany me. He is 6'/175 lbs and I'm am 6'/185 lbs. We are both in relatively good cardiovascular shape and the only significant physical variable between the two of us is the 32 year age difference. Here's the thing: every time we ride together and are on an incline ( nothing steep), I find I require a significant amount of effort just to keep pace with him. While I'm pushing pretty hard I look over at him and he looks as if he's on a leisurely Sunday afternoon outing. What really makes this problematic is that I ride a Trek FX 7.5 with 700x32's while he is riding an old Giant MTB with knobby tires the width of a Greyhound bus.

What's going on here? Am I missing something? Tires aren't rubbing on my brakes and my bike is in great shape. Any thoughts? And please don't tell me that it just because I'm OLD!!!: