I live in Niagara, you dont have much choice...pretty small place...but Dragonfly is usually a mixed crowd (17 with fake ids all the way up to some cougars 40-45...but mostly people under 25)
Theres also the new place, Vice Lounge at Thorald Stone and Montrose...its again a mixed crowd but no one under 20 usually, pretty nice place and decently cheap
For teenage and under 23 mainly theres Mardi Gras and Rumors at Clifton Hill
If you just want food, after 1am theres only a few places...strippers go to Flying saucer mostly after work (lundys lane), really other than that theres tim hortons or fast food.
If you want just a bar, theres a number of places at the top of Clifton hill which are mostly a mix of 17 year olds...again with fake ids...up to like 25/30
A bit further down the hill theres Boston Pizza and Dave and Busters...both decent and pretty busy bars
A bit further down from there, on the side with Dave and Busters theres a PizzaPizza with a karaoke place and bar, pretty good place if you like watching drunk people sing sweet home alabama
Outside the tourist areas if you dont mind driving theres a ton of places near Brock university and really everywhere...we have a lot of bars haha
If you want a list of university bars let me know and I can post them up...but theres like a TON between niagara, throld and st kitts (the university area pretty much)