What is the best variable to differentiate between 1.5 immigrant generation and 2nd generation?

I am interested in looking at the Mexican-origin population and I am trying to differentiate between the 1.5 generation (those who migrated as children before the age of 11) and the 2nd generation (those who were born in the United States of at least one immigrant parent). Could you please help me identify a variable or variables that would get at that. Thanks!

The answer to your question depends on what data source you are using and what years of data you are interested. In general, though, you would likely want to use AGE, BPL, YRIMMIG, MBPL, and FBPL. Note that year of immigration (YRIMMIG) is sometimes aggregated into broad categories, depending on the sample used, and that MBPL and FBPL are not available after 1990 in IPUMS-USA.