These wonderkids can range from the age of 16 to 19. In past editions there were occasional ones who were younger; anyone remember Ole Martin Kristiansen and Alexander Kristiansen of Championship Manager 2003/04?
To be defined as a wonderkid the football player must still have room for improvement. This means their stats will increase as they get older and as they play. The biggest stat jumps tend to be between the ages of 16 and 19. To be good enough to be classed as a wonderkid their potential must be well above 150, more like 170-200. This usually means their rating is -9 to -10; Football Manager's way of generating a random potential ability where the lower the number the better the potential. To put this in context, Jack Wilshere is a -9 while Messi was a -10.
Goalkeepers
Having given you some age brackets it’s time to break the rule. Goalkeepers mature differently than players in other positions. Many wingers like Gareth Bale and Seamus Coleman move into the full-back position as they get older; whereas keepers get better as they age and can keep going until 40 (think David James and Brad Friedel). Here’s a list of good young goalkeepers: