A competitor is considered to be a junior up to and including the calendar year in which he attains the age of 18, or 21 in the case of F2. See the table below. All other competitors are classed as seniors.

c) C.5.1 Competitor F2 Subcommittee, Poland
Amend the first paragraph as follows:
A competitor is considered to be a junior up to and including the calendar year in
which he attains the age of 18, except for F2 where the age shall be 21. All other
competitors are classed as Seniors.
Reason: F2 classes are both complex and physically demanding. In 3 of the 4
classes young people are unable to be competitive against senior pilots until after
the age of 18 yrs. Once reaching the age of 18 many good juniors are lost to F2
because they can no longer be competitive.
Reasoning given by Poland:
F2 classes are both complex and physically demanding. Young people are unable to
be competitive against senior pilots until after the age of 18 yrs. Once reaching the
age of 18 many good juniors are lost to F2 because they can no longer be
competitive.

Confirmed on the FAI web site, the age is now 21 for F2 classes

C.5.3 National team for World and Continental Championships
a) A national team shall consist of a maximum of three individual competitors, or three pairs of competitors, for each category and a team manager.
b) For those categories that do not have separate junior championships, the team may consist of a maximum of four individual competitors or four pairs of competitors for each category provided that the fourth competitor is a junior, plus a team manager.
c) The reigning World or Continental Champion has the right (subject to the approval of his National Airsports Control) to participate in the next World or Continental Championships in that category regardless of whether he qualifies for the national team or not. If he is not a member of the national team, his score will not be considered in the team results.
d) The reigning Junior World or Junior Continental Champion has the right (subject to the approval of his National Airsports Control) to participate in the next World or Continental Championships in that category regardless of whether he qualifies for the national team or not, and provided that he will still be a junior pilot when the next World or Continental Championships are held. If he is not a member of the national team, his score will not be considered in the team results.
e) Any Junior World or Continental Champion who will be too old to defend his title at the next Junior World or Continental Championships is entitled to fly in the appropriate Senior World or Continental Championship for the concerned class, within the three calendar years following his becoming Junior World or Continental Champion.