If your power training a lot, you get to keep the young player for 4 more seasons than the scout player meaning you save regular investments that could go on more scouts. Its like the durability of the young 18 year old is twice as long as the scout player as far as power training is concerned.
Power Training cut-off age is 25, that means after 25 the rate of growth begins to decline.
Michu 22 23 24 25 end of optimal power training = 4 seasons to power train effectively
Miku 18 19 20 21 22 23 24 25 end of optimal power training = 8 seasons to power train effectively
With enough growth and tokens, we could see Miku be a 9* player on a few occasions before the age of 26.
It's worth me spending my tokens power training these 2 players because a few of my other squad players like Zidane and Papaleo are now out the power training age window.