IME, around a year old, or even a little before they can breed.
How old they are from the pet shop is somewhat difficult to determine. Based on size (the usual 2" to 2 1/2" they typically sell cichlids at), they are comparable to my juvies at about 8-10 months old (quite a bit smaller then say convicts or jewel cichlids would be at even 6 months old). Though i would suspect the breeders would be getting them to a saleable size with in 6 months (??). If I went to the trouble of raising brine shrimp, I could probably grow salvini fry quite a bit faster at a young age. But IME, Salvinis do grow very rapidly from about 8 months to 2 years old.
It's not inconceivable that your salvinis could breed with 1-2 months from purchase. Or sometimes even sooner. Sometimes fish spend many months at an LFS before they are sold so are actually a little older then their size might indicate.
Likely they would need to grow some before they become sexually mature. Maybe a few months or so.
Curious as to how you are sexing your salvini. I don't see a dorsal spot/blotch on the supposed female. By the time they are put up for sale at an LFS, females usually already possess a dorsal spot/blotch.