I think there is confusion when it comes to forecast and progression.

Progression is just a % growth rate, ie the rate at which a player will train from the time he leaves school till the time he reaches his peak number( for example 75)

It is a fixed value - this is important to understand (If it changed all the time, it would be difficult to calculate)

Now, the trick is to fully understand the forecast because this DOES change. If a player is 14 or 15 and just out of school his forecast will show the maximum he can achieve. With good training (especially as a junior) he can attain this number (I dont want to call it a value because that pertains to a monetary amount). With little training or promotion too early or leaving a player unpromoted at 19 etc (low morale) or playing a player out of position - these impact on the players forecast but not his progression. If you buy a player with low morale and maybe a medium forecast (say 55), boost his morale and train him at least once a week, you will see his maximum number on his forecast actually go back UP (but not to the same amount as he was at 14)

So, saying all that you can see that age does not matter when checking forecast, and age is not in the equation for calculating progression either.
Now, forecast reaches a peak and starts to tail off. If a young player has been vigorously trained and reaches his forecast peak at 19 he will plateau for a long time (stay at that number) but then start dropping.

For more info check out this other forum thread where rebsiot and I have some fun.
http://uk.strikermanager.com/foros/hilo.php?id=254668

Cheers and hope that helps