Yes and no.
While the individual basic skills (speed, technique, positioning) are already there at the age of 18, there is yet a huge step to make. At this stage of development, players learn to utilize their skills to the maximum success and cover their deficits. This happens mainly through experience and tactical development. Even in a club with a strong philosophy through all the youth levels, the tactical adjustments to the grown-up football are enormous. Plus the mental issue you have already mentioned.