When were the 1st, 2nd, 3rd and 4th generations born for American ethnic groups?

[King of Kensington]

The terms Issei (1st), Nisei (2nd), Sansei (3rd) and Yonsei (4th) are used for Japanese American generations. 

Looking at the early 20th century wave, the majority of the Issei were born in the 1880s and 1890s, the Nisei mostly between 1915 and 1935.  The Sansei were mostly born post WWII (Baby Boomers), while the Yonsei were born mostly 1970s, 1980s and 1990s.

The generation structure is probably pretty similar for Jewish and Italian Americans from the Ellis Island wave.

It would be interesting to come up with dates and even terms for other groups like Irish, Cubans and whatever, though this is of course complicated by multiple immigration waves and the length of immigration waves.

[Brittain33]

For Eastern European Jews:

Immigration was high from 1881 to 1914 with cliffs on either side so the bands are 35 years wide with lots of overlap. 1st generation was mainly born between 1860s and 1900s, with outliers on either end who were adults arriving early or kids in the final years.

2nd generation started shortly after immigration started, with largest numbers born between 1890s and the Depression; my father an outlier born in 1942 with his father arriving as a child in 1916.

3rd generation was 1930s (Woody Allen is among the oldest 3rd generation) through the Baby Boom and into early Gen X (I'm 3rd and 4th generation born 1976).

4th generation maps to Gen X and millennials.  

This schema excludes descendents of earlier German Jewish immigrants; the refugees from Nazism; and ex-Soviet Jews.

[King of Kensington]

According to this article by Joel Perlmann (p. 56-57), second generation Irish births were born mostly between 1856 and 1886 (peaking in the 1860s and 1870s).  The third generation peaked 1887-1910. Presumably the fourth generation were born mostly in the 1910s and 1920s.

https://books.google.ca/books?id=-zC7VEqM8tQC&printsec=frontcover&dq=ethnicity+and+beyond+lederhendler&hl=en&sa=X&ved=0ahUKEwjE-8yHsZrTAhXKzIMKHZkWADUQ6AEIHDAA#v=onepage&q=ethnicity%20and%20beyond%20lederhendler&f=false

ETA:  Although Irish immigration peaked in the Famine years, as many came between 1860 and 1890, so there's another group roughly a generation behind. 

Of course this doesn't include 20th century immigration, much more skewed towards Boston and New York.

[King of Kensington]

The first wave of post-1959 Cubans was in the 1960s and early 1970s*, so you probably have a second generation born roughly between 1965 and 1995, and the third generation has begun to come of age.  The first generation from this wave is obviously quite old, a majority over 60 and the oldest in their nineties. 

* The slowdown in the mid-to-late 1970s allows for two pretty discrete generations.

[King of Kensington]

For the post-1965 immigrants, the 2nd generation is generally under 45.  Immigration levels increased through the decades, so offspring of the "pioneers" of this wave, born in the 1970s and 1980s, would be outnumbered by those born after 1990.  The 3rd generation is still, I suspect, small and hasn't yet come of age.