I am a man-child, but I'm only 28. Man child isn't my problem with him, it's the fucking laughing. I like when the people can't stay in character on SNL, when it's rare and really funny skit. He would go "How, it's Saturday!" and would start laughing. So fucking stupid.
Hell, I'll call myself a "man-child" and I'm 35!
There's plenty of great examples in SNL where one of the performers would break character and giggle, or stumble over line or something. (See the infamous first "Motivational Speaker" skit during Farley's antics and when he falls onto the breakaway coffee table. Spade and Christina Applegate. are clearly giggling over things. But those moments were rare and in pretty extreme skits with very good, solid, performers (like Farley.)
Fallon did it ALL OF THE TIME! Every single skit he'd do no matter what the skit was about or who he was co-staring with he'd stat giggling to himself like a school girl and very often flub his lines like he hasn't been practicing this skit all week. He, for me, was part of the decline for SNL.