As for how she'll mark the landmark, Swift, who is dating Jake Gyllenhaal, played coy when asked about celebrating during a Monday morning radio interview.
"Plans? For my birthday? This is it! This was all I planned for," Swift joked to Nashville DJ Gerry House on WSIX. "It snowed yesterday, you know, so that kind of limits activities. I may sit inside my apartment and pretend I'm in a snow globe."
But the singer did receive a morning serenade: Along with a birthday card, the radio show's cast and crew presented Swift with a rendition of "Happy Birthday."
Swift says despite hitting the milestone, "I don't feel older all of a sudden. It's a state of mind thing, how you see the world, being excitable, staying excitable and impressionable and doing your best not to become jaded or bitter. I think keeping your mind and your heart open keeps you young."
Asked whether she felt overwhelmed by all her life now holds – including news that her latest CD, Speak Now, has already gone triple-platinum – she said, “Things feel really good on this birthday, really great in fact."
She says she wrote "21st Birthday!" on her calendar "really big," but "that's about as far as it went for the day."
Swift, however, did reference some possible evening celebration, saying with a laugh, "Later on I might have plans, but not really during the day."
Though she’s recently spent several days in L.A. with Gyllenhaal, she Tweeted that Sunday night she stayed home in her new Nashville condo, "dancing around my room alone to @pink's greatest hits cd," after two band mates had dropped by with birthday hats and a pizza lit with 21 candles.
Reporting by KAY WEST