It depends entirely on the individual rat. My girls who are about a year old get on fine with the 2.5 year olds, but really don't get on with the rats who are below 3 months. One possible reason for this that has been put forward is that maybe the 1 year old rats see the younger ones as breeding competition.

Yours are still fairly young so they should be fine together eventually, just keep trying.

Peace
Akai-Chan