If I'm over 25, is it still too late for me to "learn" French or can I just watch French sitcoms for like a year and be up-to-speed?

No and no. It's not too late, and just watching sitcons won't teach you enough to start speaking. But if you just start actual studying and practicing you can learn it just fine. Watching TV can be used to practice listening, but on it's own it's not enough unless you're a wonderkid.