Why does not U18 players don't get "played in 24hr" experience for training when playing u18 league? 

I think you already answer the question.

its logical that youth what not play in youth league match dont get exp.

And if by "played in 24hr" you reffer to trainning bonus, they do not get that in the Youth League. The only thing you can get in the Youth League is very very little match experience for players that do play in the Youth League matches

If u click on the "info" button in trainingovervieuw with under 17 yo it says played last 24hr green v or red x so it is a fair question if u ask me what matthew asks. If last played 24 hours only counts above 18 years old or only when a player is promoted first it sounds like a fair question. I dont see anywere different rules or detailed rules about it. If a player is 17 and he plays youthleaque that bonus is not applied and he gets only 90% when have 5 trainer, 5 star pull and without mentor.When the same player is 17, is promoted to main team and subs in 15 minutes that bonus does apply and he gets 112.5 % at training.