Artist maturity

I turn 40 tomorrow (April 2) and it has me thinking about not only physical maturity but maturing as an artist.  Is my art growing, changing?  I think it is in different ways.  The way I am creating bevels now is better and different than years ago. My paintings have matured over the years.  My painting instructor/adviser in college once told me that it takes 10- 15 yrs to really develop your style and then many years of continued growth.  What do you think?