Its the jump/lift that's uncontested.
Once the ball is secured then its back to contested.
That's how it was interpreted for my U15s last year where were had uncontested lifting. However looking at the U14s regs below c and f would imply that the lineout (not the catch) is uncontested until it is over in the usual ways.
15. The Lineout:
a) The lineout will be awarded from the touchline level with where the ball crossed the line, to the opponents of the player who touched the
ball before it went into touch.
b) The ball must be thrown down the middle of the channel. If the throw is not straight, a lineout will be awarded to the opposing team. If this throw is not straight, a scrum will be awarded to the (team originally awarded the line out) on the 15m line.
c) The lineout will be uncontested by up to 13 players from each team
d) No lifting or supporting of any kind is allowed.
e) Players not involved in the lineout must remain 10 metres behind the mark and must do so until invited forward by the referee.
f) The lineout ends when the ball or a player carrying it leaves the lineout or the ball goes beyond the 15m or into the 5m channel.