I'm gonna say I am in my 20's but I'm recommending places that I've seen a healthy mixture of people of all ages so you guys don't feel ancient!
As SPG said Ye old trip to jerusalem, either hope its sunny or quiet as it is very small inside its built in the cliffside under the castle.
Also second canalhouse, possibly my favourite pub/bar in the city, the booze selection is tremendous and the canal goes into the pub. Note however Canalhouse is always rammed and its on the outside of town centre.
Penny lane is a really cool bar, its funky has hockey tables, foosball tables and other arcade style machines as well as your normal seating. I've seen plenty of older groups/stags/hens in there too.
Pitcher and piano is in an old church which is pretty cool, its just a chain though so nothing special to say about.
Das kino, its got 3 pingpong tables dotted around. Booze is good but you will be occasionally struck by a stray ping pong ball
Alchemist, its a chain you've probably been to one before but its nice in there and they do an amazing old fashioned cocktail.
Bunk, there is 3 of these around the city, I'm not sure how many times I've been now but its been a lot! Its a cocktail and wings bar, before 10pm cocktails and wings are 2 4 1. Well worth a visit to one of them. All have cool interiors.
Boilermaker, its a speakeasy bar hidden out back of an old boiler shop. Its cool, not worth a whole night but if you guys are planning to visit quite a few places I'd consider throwing it in the mix.
Not so much a bar but theres a funky underground crazy golf at the cornerhouse called lost city, it has a bar down there too just thought I'd throw that out to you if wanted to do something other than drink.
Other than that I'd just say avoid any bars by the cornerhouse, they are all big chains like revs, loud, poorly furnished and there is so many better bars in town.
There is plenty more places to recommend but I've tried to be generic, if you let me know the kind of night you're after I can recommend more if needed
EDIT: Also I would recommend Annies Burger Shack over BOHNS, BOHNS is good but annies is better IMHO