From jw.org, here is the new definition of the Generation from the April 15, 2010 Watchtower.
Article: Holy Spirit’s Role in the Outworking of Jehovah’s Purpose
13 Third, holy spirit is at work in bringing Bible
truths to light. (Prov. 4:18) This magazine has
long been used by “the faithful and discreet slave”
as the primary channel for dispensing increased
light. (Matt. 24:45) For example, consider our understanding
of those who make up “this generation”
mentioned by Jesus. (Read Matthew 24:32-
34.) To what generation did Jesus refer? The article
“Christ’s Presence—What Does It Mean to You?”
explained that Jesus was referring, not to the wicked,
but to his disciples, who were soon to be anointed
with holy spirit. Jesus’ anointed followers, both
in the first century and in our day, would be the ones
who would not only see the sign but also discern its
meaning—that Jesus “is near at the doors.”
14 What does this explanation mean to us? Although
we cannot measure the exact length of “this
generation,” we do well to keep in mind several
things about the word “generation”: It usually refers
to people of varying ages whose lives overlap during
a particular time period; it is not excessively long;
and it has an end. (Ex. 1:6) How, then, are we to
understand Jesus’ words about “this generation”?
He evidently meant that the lives of the anointed
who were on hand when the sign began to become
evident in 1914 would overlap with the lives of other
anointed ones who would see the start of the great
tribulation. That generation had a beginning, and it
surely will have an end. The fulfillment of the various
features of the sign clearly indicate that the tribulation
must be near. By maintaining your sense of
urgency and keeping on the watch, you show that
you are keeping up with advancing light and following
the leadings of holy spirit.—Mark 13:37.