I don't know the answer for how many known characters are divergent, but I can answer the second part about the age difference between <PERSON> and <PERSON>.
We know <PERSON> and <PERSON> are a little less than a year apart.
"<PERSON> is not quite a year older than I am, so we are in the same year at school." Divergent Chapter 1
Their mom could have had them both in a short time.
Women who are breastfeeding a newborn are very unlikely to conceive, and most women who aren’t breastfeeding won’t start ovulating again until 6 weeks after giving birth. But it's possible for a woman to ovulate in less time. (source)
We don't know details about breastfeeding babies in the Divergent universe, so that aside, it's possible that <PERSON> gave birth to <PERSON> and conceived <PERSON> 1-2 months later. Then <PERSON> would have been born 9 months later, making her 10-11 months younger than <PERSON>.
The dates of <PERSON>'s and <PERSON>'s birthdays aren't mentioned in canon, but being almost a year apart and in the same grade in school makes sense if <PERSON> has a summer birthday and their parents decided to hold <PERSON> back a year before starting kindergarten (or whatever the first year of school is called in the Divergent universe). I'm also assuming that since they live in Chicago, their school year starts in late August/early September.
Hypothetically, let's say <PERSON> was born in July and <PERSON> was conceived in September (2 months later). 725
That puts <PERSON>'s birthday in June, 11 months younger than <PERSON>. When <PERSON> turned 5 in July, his parents decided to hold him back a year for kindergarten. The following year, <PERSON> turned 5 in June, <PERSON> turned 6 in July, and they started school together, in the same grade, in September.
Why hold <PERSON> back a year but not <PERSON>? There are a few possibilities, but I think the most likely is because in general, girls tend to mature faster than <PERSON>. It's possible their parents thought <PERSON> was ready to start school at age 5, even though they didn't send <PERSON> at age 5. (And <PERSON> being divergent may have also made her cognitive skills mature faster.) 725