Neuroplasticity is supposedly on a constant decline after childhood. Also processing speed, working memory, long-term memory. Offset by increased experience, of course.
While true that neuroplasticity is probably declining after reaching age of 20-21 (?), that doesn't mean much. People can learn new things during their lifetime. It's not like when you have a 21-year old, they are learning at the speed of light, whereas a 40-year old takes ages to learn the same tech.
