My experience learning a new keyboard layout was at age 19 an I was not typing for more than a few hours a day.
It takes much longer to both break old habits and form new ones from older ages. If you have been typing for 20+ years in QWERTY and use a keyboard everyday professionally learning a new layout will likely be a very hard and frustrating task. Every time you switch back to QWERTY you are basically reversing the practice you have put in. If you are typing for a profession you likely won't want to suffer months of half your former typing speed or less until you catch up.
It takes much longer to both break old habits and form new ones from older ages. If you have been typing for 20+ years in QWERTY and use a keyboard everyday professionally learning a new layout will likely be a very hard and frustrating task. Every time you switch back to QWERTY you are basically reversing the practice you have put in. If you are typing for a profession you likely won't want to suffer months of half your former typing speed or less until you catch up.