I'm 34 as well, and I'm amazed at some of the things my 20 year old self was able to solve (mostly math and physics related stuff). There's a belief (apparently not yet backed by research) that math skills degrade after 35, as this article discusses and tries to disprove:

http://www.massey.ac.nz/~rmclachl/overthehill.html

However, I don't think it's slowing me down, I'm still capable of learning and producing, and I've learned lots of stuff to "work smarter". I do believe I'm doing myself a disservice by not keeping in shape, and that might be part of my perception of having less energy (but not less enthusiasm).