Define "younger". I'm 30 and although I grew up with TNG (which I now see as extremely flawed) I love
TOS. I'm sure some old timers would be furious that I enjoy it's dated campiness, or that I think it's ever so slightly tongue-in-cheek. I love the characters and their interactions (which can carry even the worst episodes), it's entertaining, colourful, fun and it created a wonderful universe which I'm thoroughly enjoying exploring anew in the reboot movies.
All that said, coming to it many years later I'm not sure I can separate the 79 episodes of TOS itself with what the animated series, movie series, novels, comics and endless fanfic added. They made the world infinitely deeper and richer, and the novels and fanfic fleshed out the characters like a TV series never could.