Well neither was really explained. It happened in season 2. It was intended to make the teams in the game have players to spend there money on. Back then you hardly ever saw a player with strong skills being sold.
As far as I remember there was no mention of when this speedy training would end.
Correction: It happened during the All Star Break during season 3. The increase was said to be a bit faster than 150% (but less than 200%) for all players, and about 200% for the "youngest players". I'm going through the archives to find a link now.
Found it:
(6866.1) There sadly is no complete quoting of the news item, but many elements were discussed. BBs Domenico & Charles both participate in this thread.