Additional factors:-
Age
The players training speed will slow down by a bit every year, for example, at age 18, his training speed is multiply by 1, then at age 19 it is multiply by 0.99, 20 is 0.98, and go on the same way every year.
From many people provided information, it is said that the skills will start to drop by the age of 32 onwards but just for your information I have a friend who have a player which is 33 yrs old right now, did not experience a drop in skills level before eventhough he did not train this player at all.
Potential soft cap
I'm not very sure about potential soft cap but many people says it is depends on salary, someone told me that when the player reach its potential soft cap, he'll need 10 weeks for an increment of skill eventhough the player is going through a one position training every week and get full training minutes. Thats a big range to notice, a 2 weeks and 10 weeks.
This is the potential soft cap table from one of the sticky threads:-
0 Announcer: < 6k
1 Bench warmer: < 8k
2 Role player: < 13k
3 6th man: 15-18k
4 Starter: 26-30k
5 Star: 45-50k
6 AllStar: 60-80k
7 Perennial allstar: 95-140k
8 Superstar: 140-220k
9 MVP: 215-315k
10 Hall of famer: 500k+
11 All time great: 1M+
Thats all that I recall for now, will add in as a reply if I recall more of it.