Google the function read_csv and find a way to import the Barro-Lee dataset into your R session. Similarly, install and load the package readstata13 and look up the function read.dta13 to load the Penn World Table dataset. In the Barro-Lee dataset, filter out so that you only consider people between 20 and 24 years old, using the variables ageto and agefrom.
The advice there is pretty good. If you google r read_csv you will get the documentation for the read_csv function. Note the examples at the bottom help you load that data.
If you're using RStudio, it has a handy GUI widget to help you load data into R.