Steph, I believe, was older. It was mentioned in an early Robin issue.
Cassie, I think, is Tim's age or a year younger. She was introduced as a 14-year-old, and this was in 1995 or so. Tim had been Robin for a while already, and he started when he was 13/14, so he's either her age or a year older. Conner's a bit older than Tim, being created at 16, but I'm not sure how much time he lost in the period where he couldn't age, and the year of being dead probably puts him a bit younger than Tim, about Cassie's age now.