[R] work with a subset of the dataset

[manu79]

Hello,
I have a big dataset with many variables and I would like to consider 
only the rows in which there is a specific value of a variable.

I make an example for explain what I mean:
I have 5 variables describing a person: age, sex, weight, colour of 
hair, colour of eyes.
I have 1000 rows (1000 persons) and I want to consider only the persons 
whose age is between 20 to 30. How can I do?

Thank you very much
M.