Nope, the good ones aren't all 'taken' by 27 for a whole bunch of reasons:
(1) This business of 'putting a ring on' a guy/girl when you're pretty young (before the 'good ones get taken') is predicated on your ability and good judgement at, say, age 18 or 19 to select a spouse with characteristics that will make you happy when you're 25, 37 and 78.
(I don't know about you, but OMFG-he's-so-hot was my main criteria for selecting boyfriends until I was about 23. Any guy I dated before then? I don't think I'd be able to find things to talk about over a 20 min coffee. Marrying that young, for me, would've resulted in divorce).
(2) The average age of first marriage is bimodally distributed -- it's around 3-4 yrs after you finish your formal education, i.e. 21 for HS grads, 25 for college grads, way older for those who spend a billion years in grad school. So if you're still in grad school and still single, you are likely surrounded by many single people your-ish age who share your interests.
(3) The 50% across-the-board-divorce-rate -- even if some awesome person is taken/married before 27, there's a 50% chance they'll be single again within 5-7 years (point in a marriage when divorces are most likely to happen).
(4) The anecdotal but exceedingly high divorce rate among folks who get married at 21/22 all of 48 hrs after college graduation. 7 of the first 8 weddings I went to ended in divorce within 18 mos (the most spectacular flamed out within 3 mos). The moral of the story? If you're 21/22 and in college AND engaged? Be engaged for a year or two before tying the knot. Real life is almost NOTHING like college, especially if you lived in a dorm for four years and your parents paid your tuition.
(Though my college BF proposed, I said no and we shacked up in sin and it was the BEST DECISION EVER because we broke up 4 months later. Having an ex-live-in-bf is preferable -- and also about $25k cheaper -- than getting married and then divorcing 4 mos later).