If we go by 'canon', a newspaper that flashes up during Reichenbach puts John's age at 37, and if you add to that with the (two!) original canon theories about Watson's birthday, he'd either have just turned 37, or be about to turn 38. Which would put him at 35-36 during Series One. On John's official blog, he refers to Harry as being 36 during Series One. Which would make her either slightly older, or the same age. But I think Sherlock would have mentioned if they were twins. And canon isn't exactly brilliant with keeping itself logical. If you go by John's CV, he's 28 in Series One, which, despite Martin Freeman's spritely appearance, is stretching things a bit.
So, I guess it's basically up to me what I want to make her. She's always been older in my mind, but I was just curious to see what everyone else thought, or if I've missed something.