I've been to both Jillian's and Jake Ivory's recently, on several occassions (I work nearby). Being in your age group, here's my take on things.
Jillian's
- Mix of ages, from 20's on up especially in the pool hall area. The floor with video games tends to attract a younger crowd. The bartenders mix good drinks and the food is not bad. They offer calzone, salads, chicken fingers, etc. One co-worker described it as "Chuck E Cheese for adults".

It's a fun place.
Jack Ivory's
- I was there on a Friday night in the summer. That said, it was very crowded with young (20's) and drunk people. There were tables there but you couldn't sit down, given the crowd. It looked like a "rave" going on in there. We were already drunk when we got there so it didn't much matter and we joined in the fun and singing. I doubt I'd go back, unless I was drunk again.
Weekends
- Landsdown Street is all clubs and bars. Since many College kids and 20-somethings are in the area, this is their "hangout". It can get very crowded and wild down there some nights. If there's a baseball game the area is packed. Watch the Red Sox schedule and go down there on a weeknight if possible, unless you want to fight your way through the baseball crowd and half naked club babes.