When the Z-Gang realizes that the have to train
as hard as they can before they take on Cell for
the Cell Game, Trunks and Vegeta go back into
the Room of Spirit and Time. After being
in there for another year, Trunks comes out of
it as a 20 year old. This version of Trunks
makes his appearance during the Cell Game.
It is also at this age when Trunks is killed by
Cell. Luckily, he gets bought back by the
Dragon Balls when the fight is over. During
DBZ Movie 8, it is also assumed that Trunks is
20 years old. When the Cell Game is complete,
Trunks returns to his future for good, where he
kills the Androids #17 and #18 of his time period.
21 year old Trunks