Radix:

Females, mine usually want to start breeding between 18 months and 24 months.

Males, I've had males as young as 1 year old try to court females. I've never used a male that young before so I not sure he would have locked up with her. Most of my males are ready to go at 18 months.

As a rule, small females put out small groups.
I just had a first time(small) mom give me 4 babies(2 alive, 2 stillborn)
The more mature females have larger size babies and groups. This is working solely with my radixes.