I've heard age slows down training 10-15% which I feel is rather high.
To my knowledge, no-one has ever bothered to conduct a systematic study of this. And having in mind how many other things factor into training speed, whoever told you this was probably pulling numbers out of their rear end.
Yup.. maybe --- plus, they say now the salary won't determine the abilities of the players, so... maybe multiskilled players will have a "bonus" - meaning that the salary of having a multi "prolific" guard (lets say in 5 skills), would have a somehow lower (regarding quality) salary that a guard with with just jump shot and shot range up high (maybe even with these two skills just a notch higher than prolific)....
It would be somehow of a "punishment" for 1 dimensional players and their trainers..