I don't think it's about age. I am not 20-something, but I love texting.
Communication with Foreign Friends:
I used it to communicate with my good friend who doesn't speak English well. I couldn't understand her via phone, so we'd text one another.
Privacy for Gossiping:
I like to complain about my co-workers to my mom, but since she refuses to learn to text, I have to call her from outside to gossip.
Getting to the Point:
Via a text message, there's no "What are you doing? (long pause)" chit-chatter. It's a friend saying - "Hey, I'm on the way." Reply - "cool, c u @ 3" Boom, that's it.
Time to Lie:
On the phone, it's harder to lie. Via text message, you have time to THINK about your lie, then type it out. No stuttering on the phone or nervousness in your voice, y'know.
Texting RULES!!