I have a T-shirt with all the Senior winners 1907-2012 on the back, and if things printed on T-shirts can be believed, then this year was the 95th.

1907-2015 would be 109 races if no years were missed. The missing years according to the T-shirt are:
WW1 1915-1919 (5 years)
WW2 1940-1946 (7 years)
2001 FMD outbreak
2012 cancelled
So a total of 14 years without a senior.

According to the TT database 1911 was the first year there was a race called the "Senior TT". 1907-1908 had single and twin cylinder races, and it looks like 1909 and 1910 had only one race. Apparently the current Senior trophy was awarded to the winner of the single cylinder race in 1907. I'm not sure if it was considered the trophy for that specific race, or if it was for the fastest overall - the singles were faster than the twins, less to go wrong I suppose!