Though I congratulate Anu and AWAD heartily on their sixteenth anniversary the relation between 16 and the given words is pretty loose. Also none of the online dictionaries give a life sign for the word sexdecennial, which at first sight I interpreted as 60, while googling the word brings up: about 121 for sexdecennial. One more (122) after I googled it a second time.

Is the word "dictionatical"? (to use a tsumesque appraoch).

I don't think it's dictionatical if it isn't in any dictionary. It is based on the structure of quindecennial.

Not to mention undecennial and duodecennial.

60th anniversary would be sexgennial. Jocular words like these (see this list) assume a familiarity with Latin. I still remember fondly my hometown's sesquicentennial in 1973 and then a few years later a professor noting the sesquimillennial of the fall of the (Western) Roman Empire.

Can't say my familiarity with Latin goes very far. I had already found another list but it looked like my worst nightmare and I dared not mention it. It's o v e r w h e l m i n g. Crushing. all of it Someone's real labor of love.