I don't know how old I was when playing Monster turned from fun to silly and I outgrew it, but I'll bet I know when we first started playing! Say, about 16 months or so?
Yes, Sophia has discovered Monster. And actually, she made it up today after nap.
Our living room is set up in such a way that she can circle all the way around the couch, and sometimes she likes to hide out behind it (usually to poop in privacy). Today, though, she just wanted to circle around and around, and it was the most fun if I chased her a little bit, threatening to tickle her (and sometimes actually tickling her) while making growling sounds.
Of course, we did the fun version of Monster, not the scary version. When the chasee is only 16 months old, the monster still smiles a lot.
Knowing how I used to love playing Monster, I expect to get a lot of mileage from the game. Of course, what I never realized back in the day is that it is just as fun to be the Monster as it is being the "scared" little kid.
Now back to playing Monster!