I'm assuming that Faith is a year younger than Buffy, because that's always how I pictured it. They could actually be the same age, so that would kick her up a year.
BtVS S3 aired '98/99 - Faith is 16, turns 17 and becomes Eeeeeevil and then goes all coma and gives Buffy a prophecy dream clue
BtVS S4 & AtS S1 aired '99/00 - Faith is 17 (hey, did she miss her 18th birthday whilst comafied?), turns 18 and goes to jail, does not pass go
BtVS S5 & AtS S2 aired '00/01 - Faith is 18, turns 19 (And it is established that she and Angel are conversing, and can I say how much I loved it when Angel went to see her?)
BtVS S6 & AtS S3 aired '01/02 - Faith is 19, turns 20 (Appears in neither show, which is why these are my least favorite seasons. Kidding! I have less shallow reasons for my strong dislike.)
BtVS S7 & AtS S4 aired '02/03 - Faith is 20, turns 21 (Faith/Wes forevah. Faith survives Sunnydale yay and may or may not have an established relationship with Wood.)
AtS S5 aired '03/04 - Faith is 21, turns 22 (Angel dying? Wes dying? Hello angsty allusions.) Meanwhile the actress stars in a doomed Fox TV show that didn't get very good reviews from what I heard.
2004/2005 - In my head, Faith travels around, doing the Slaying gig on her own as a freelancer. Her relationship totally didn't work with Wood, not like that surprised her. Also, Tru Calling gets canned. Faith is 22, turns 23
2005/2006 - SPN - Faith would be 23, turning 24. She and Dean fuck many times over and it awesome.
All that is just for my own peace of mind.