OK, here it goes.
First of all, BACKUP YOUR SAVE DISK. Screwing things up and making it unreadable is very easy.
-Write down your player manager's attributes (Age, Height, Shooting, Passing etc.)
-Download one of the many free Hex Editors that you can find on the internet (I use Hex Workshop
-Use it to load your savedisk ADF.
-Type Ctrl+F and enter your player manager's name:
You will most probably find him in more than one place, especially if you keep multiple saves in the disk. Look for a sequence of numbers in the vicinity that match your player's attributes. Don't look for Pace, Stamina etc. because those could apply to multiple players if they're all near 200. Look for Shooting, Passing, Tackling. Of course everything is in Hex, which is to say that 178 will appear as B2. Most programs have a built in Hex calculator that will come in handy.
As soon as you locate your current PM,
look for his age byte (if he is 37 years old, then in Hex look for 25) and change it to 12 (which is 18 years old). Save the file and if you did everything correctly, your player manager will be young again.
Note that you must shut down the Hex Editor before attempting to load from the disk, it won't work if it's still open.
I hope that helps.