I've never found a site that says how old Denise is, but I'm guessing eighteen - twenty 'ish. Tron and Volnutt are both between fourteen and fifteen, so it'd seem like Denise should be young also. But in order to be a Cop, you've got to be eighteen, so she's got to older than seventeen... Then again, not everything made a lot of sense in Megaman Legends, age wise.
Never have I found an answer to the second question, either. But I'll infer like I did my first answer; Megaman is fourteen 'ish, and I was about 5'11" when I was fourteen (But I was on the taller side). Megaman seems like he's a little on the shorter side, so maybe 5'7"? You could assume that all the adult male characters are the average height (5'10") and the adult women character are the average female height (5'4").
If no one else answers your last question, I'll get back to you once I finish Megaman Legends 2.
It never gives an exact date, but according to "The MegaMan Knowledge Database" it happens after the Master dies on Terra, Trigger tried to shut down the Master System, and Data was created; And takes place before Terra is flooded.
(Most of) My answers are based on estimates, but I hope I helped answer the questions a little bit.