KöMaL 2016 C1391

Ever since his birth, Martin gets as many decks of 32 cards for Christmas as the number of Christmas days he has seen, including the current one. One year, on the second day of Christmas (that is, on his birthday) he decided to build a specific castle out of the cards he had received. The lowermost level consisted of 216 cards, and every consecutive level had 8 cards less in it than the level below. How old was Martin if he managed to build 16 levels?