Regarding the IRB view of participants in research who are the groups of people that the IRB considers vulnerable for

Regarding the IRB view of participants in research,
who are the groups of people that the IRB considers vulnerable for research participation in the 45 CFR 46?
In other words, what are the identified vulnerable groups of people for research that the IRB will focus on?
Please answer these questiosn separated and use at least 2 sources no later than 5 years.