I've seen a player history somewhere in the forum where i could see that, at the early ages, the player developed most physical attributes. Around his 20's to 23 the player had a big boost on technical and after that the big boost was mental.
I think that most of the players should build a good physical skills, or else i would never pick them to play (dont want to have a 20 technique player with 8 of stamina in the midfield). After that i will try to get the technique worked out when its time for them to play more often (from the 19's to 22's). Finally, the mental attributes will come with time and age by themselves.
Of course i always do this on players that have already good attributes, specially pace stamina and acceleration, determination composure, decisions and anticipation. The technical ones its according to the position, but always above 13/14.
Hope it helped