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.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."