If your buying an 18 year old vs Scout to not power train initially and looking to 50 greens a season power training to keep the players ability up, then just get the Scouts, the Scout will have that special ability while the 18 year old will need to be trained to high-end 6* and get the special which will cost just about 225T to start off with.
22 year old Scout + 3 seasons quality insurance (when 23,24 & 25) = 200T and for 8 seasons with 2 scouts 400T.
Young player training to get to scout level + 7 seasons quality insurance (when aged 19-25) = 225T + 350 = 575T.
The crossover point of the investment is when choosing to buy a scout and planning to invest 175T afterwards for power training and then 50T for 3 seasons quality insurance afterwards. 375T tokens on 2 scouts for 8 seasons (buying your 2nd scout when the 1st one reaches age 26) is about equal to 1x 18 year old for 8 seasons. If your planning on investing more than 750T for power training a single player over 8 seasons then the choice is the young player.