Not sure that this statement is provable. Why could a player not become more 'clutch' as they go through their careers, especially as they move from rookie year to becoming a veteran?
I guess you could say that it is an attitude or a natural inclination, but that is an opinion. My personal opinion is that as a player gains experience, they could become more comfortable in late-game situations and get better at performing in them.
Just my 2 cents. (I wish I knew how to make the cents symbol instead of writing the whole word.)