Re: Is she old/big enough?
I would wait until she's at least 20 pounds and around 3 years old or so. A bigger girl is going to give you a bigger litter, and she'll probably throw fewer slugs. It's very stressful on smaller girls and can be harmful to them.
Don't push her... get some weight on her, give her another year. Find yourself a good looking guy, and then you'll be set.
EDIT: Forgot to mention, weight, not age, is the most important thing.
Re: Is she old/big enough?
She should be at least 3 years old, 6ft+ and well muscled.
Sure you could breed her, but I wouldn't just breed her to any male you can throw at her. Think it through and get her a worthy male you really want to pair her with.
It's not too late to breed her this year, but I would just wait. I would give her this year to pack on a few more pounds and age a bit more. She'll reward you with a better litter next year and will recover nicely.
Re: Is she old/big enough?
yea around after around three and a half years, 6+ feet,and a healthy body, she will definetly be mature to mate!
Good luck in the future!