There's quite a few 22 year olds in my class, so that would be before graduation. But there are also many of us who took a year or years off between. It's not really an advantage either way.
The average age of an M1 student is 24. I would assume most people graduate from undergrad at about 21 or 22. Thus, I think most get accepted after graduation.
You don't get admitted. Matriculation is contingent (usually) upon the completion of a degree. The school has a good faith assumption you'll be receiving a BS/BA/MS/etc.
I'm a senior holding an acceptance and fighting off senioritis so I feel you. But, you wanna do well either way if only for your self and your pride. However, the line between an A and B has become very blurred for me....