40 isn't senior. It is 40. And the new leagues are 18+, 40+, 55+ and in the Southern Section yes they got the extra age group for now.
"Senior" was 50+ and the term "Senior" was outdated. The USTA dropped the terms "Senior" and "Super Senior" as those terms were ridiculous to many.
So yes, 50 is senior to 18, and to 49.
Getting older might suck, but the alternative is much worse. James Dean would probably agree.
My body feels better at 45 with two titanium screws in my back, it than it did when I was 37 and not working out as much. Age is a number. It doesn't define one unless you let it.