For instance, when did this current one begin, when will it end?
There is no set length and definitions vary widely.
Ah, so what sparked Generation Y?
I always thought it was 30-40 years.
Tough to say, OP, as it varies according to culture.
For example, My SO and I recently moved from DC (where the average couple is 35-40 when they have children) to a rural area in California.
In a few neighboring towns (read: predominantly Hispanic), there is a teenage pregnancy epidemic and lots of 36-year-old grandmothers walking around.
Standard demographic measure is the average age of women at childbirth in a given population, where the women in question have all become infertile (conventionally, reached age 45). The population might be a subset of a larger population, as in R4.
How Long is a Chinaman.
I always considered a generation 20 years or so. When one generation starts reproducing.