They say in 1795 that Barnabas and Jeremiah were nearly the same age, based on Jeremiah's gravestone that makes them both 32, I don't think JF could pass for younger than that, though he did look younger in the flashback.
Angelique is 21, as Nicholas later says she was born in 1774, so I assume Josette was, too.
As for Naomi, I guess we can assume she was in her late teens when Barnabas was born, and that Sarah was a miracle, change of life baby, as was she not 10 in 1795??
Not sure about the ages of the modern day Collinses..