Flack seems to be the older one just because he's so much more mature than Danny is, and he's always looking out for Danny, but I know they're also very much supposed to be peers, so it could be that they're the same age.
Flack is pretty much been great in every ep he's been in, but I guess my favs would be:
The Fall--obvious reasons, it's our first Flack-centric ep
On the Job--The way he took care of Danny emotionally (or tried to) was the first time we really saw the caring, emotional side of Flack
Trapped--Flack had some great one liners in this one, and really, I love it when he saves Danny's ass.
Super Men--Flack's line, "Sometimes brothers fight" is still one of my favorites ever
Consequences/Sweet 16/Past Imperfect/Comes Around--The conflict arc between Mac and Flack is one of the deepest ever done on the show.
Snow Day--Because who doesn't love Flack the hero?
Time's Up--Flack as James Bond? Yes please!
I can't put All Access on any favorites list of mine because of how much I hate that ep, but Flack was great in it, and it really showcased how good he is at reading his friends and handling them the way that will be best for them. He's sweet and consoling with Danny when Danny is upset or in danger, but he knew not to condescend or pity Stella in this ep. Eddie and Melina were so fantastic in that.